why don't people say mega meters

Familiarity I guess. Mega isn't really a widely used prefix outside of computers. We even say tons instead of megagrams.
- And yet we say megatons.
  
  But only in regards of nuclear bombs. Maybe it's because of the scientific origins of these fields. Probably the same reasons why Americans measure firearm munition in mm.
  
  Instead of teragrams.
- megaton, megawatt, megapascal, megacandela (for military flares), megahertz, megajoule, megaohm, megabequerel
  
  Megatron
- To be fair, mass is weird because the base unit is kg (yes, the name includes a prefactor). I have no idea how they managed to fuck it up that badly.
  
  Apparently they were going to use gram for what we now call kg, but decided to make it kg. I expect it's because we used grams so much for food.
- Yes, I think it's a question of use. I can't think of many examples where you would quickly need to know the measurement to the nearest Mm. Maybe if for some reason you deal with a lot of lunar orbits? Diameters of exoplanets?
  Any earth distances we need to know with greater precision, and any stellar distances are probably better measured in light years, etc.
  
  Odometer readings

Megameters sounds scary and I don't want to alarm the people I'm talking to.

On Earth it’s just not needed. In nearby space it could make sense — distance to the Moon is 369 Mm. Distance to the Sun 149 Gm. But people aren’t good at visualizing the difference between kilo-, mega-, and giga-. It isn’t obvious from those numbers just how much further away the Sun is.
- For interplanetary space and beyond the time it takes for light to cross the distance makes more sense, I'd say. The moon is about half a second away, the sun about eight minutes, Voyager I a bit less than twenty hours, Alpha Centauri or Barnard's about four years, and so on...
- Would it surprise you to know that there is a unit of length that is specifically the distance between the earth and the sun?
  It's called Astronomical Unit.

I do. Unfortunately, I don't have many opportunities to do so. Which may be the reason why people don't say megameter.

That unit is used a lot in the space game Elite Dangerous. Never saw it used before that, but it made sense because it's the next jump up in large units, and it also helps keep the UI clean looking.

I wish we would. It sounds awesome to say.
- Be the change you want to see!
  Hopefully, you have many opportunities to use it.

Jokes on you elite dangerous uses Mm/s for lowest speeds in supercruise before it changes to "c" for relative to light speed

As a KSP player I use megameters all the time lol.
- Ah yes, Krazy Soup Plantation

Well mainly because where we might need to use these units, we have more standard non si units, we use AU, Light years and Parsecs where Megameters, Gm, TM etc would be useful
- There's also scientific notation which eliminates having to use these prefixes so you can more easily compare and manipulate numbers.

nobody will stop you, i've seen some publications use gigagrams instead of thousands of tons
- weirdly enough SI unit for mass is kg not grams
  
  I've always found that strange. I guess a kilogram is a lot closer to "human scale" than a gram, maybe that's why they picked it.
  
  My physics teacher once told us that this was due to the influence of disciplines that calculate with huge masses, say in astrophysics the weight of a planet or the the amount of oxygen within it. Don't know how much of it is true but the basic tenet of everyone preferring the numbers that they work with on a daily basis having as few prefixes as possible as it makes mentally handling and remembering them easier.

Aside of kilometers there used to be "myriameter" (a myriad meters = 10,000 m = 10 km).
Fun thing, in Sweden they use mil for 10 km. In Finland there's peninkulma for 10 km, but it's very archaic.

Scientific notation for everything: 5 x 10^6 m. Seriously though, I think it would be easier to think about it in megameters or gigameters if it were more standard to do so.
- Can we do 5E6 like on the calculators? Is that common enough?
- yeah but sadly not enough people know how to read that. sadface.

We already have this in Sweden. 10km in Sweden is 1 mil (Swedish mile).
When we sell/buy used cars and other types of vehicles we always count the mileage in Swedish miles.
Kilometers work but is just absurd when you start talking about 100k+ kms.

Main reason is nearly no one needs to measure things in megameters. Megameters would be a unit to measure the diameter of planets in, maybe the orbital altitudes of some moons. Our moon for example is ~384Mm away. Distances between planets, distances between stars, and distance between galaxies are many, many orders of magnitude farther than that.
As most of us rarely travel more than 1,000 kilometers very often, it's the biggest unit most people are familiar with on an intuitive level.
I'm still convinced people don't actually use the metric system's power of ten design. Like no one uses centigrams or kiloliters either. They've picked out units that are pretty close to the ones in the Imperial/Customary system, kilograms are used instead of pounds, grams are used instead of ounces, kilometers are used instead of miles, meters are used instead of yards, centimeters are used instead of inches, millimeters are used instead of sixteenths of an inch and so on. Want to confuse a European? Draw up some blueprints in hectometers.
- Your assumption that an imperial system is some kind of default tells us more about your limited worldview than about measurement units.
  
  It's a default because its older.
- Actually, size of a plot of land is often talked about in hm2.
  So no, it would not confuse a European. Because the system is easy to use.
- When measuring lengths and time, metric units are super useful. Micrometers, Millimeters, Centimeters, Meters and Kilometres are easily and often converted in a lot of technical jobs.
  Same with nanoseconds, milliseconds and seconds when dealing with time in simulations or other computer programs (e.g. game development).
  Milliliters, Centiliters and Liters are commonly used in cooking.
  What's wrong with hectometers? I don't know how often they're used but dealing with a factor of 10 or 100 to the next "regular" unit (meters or kilometers) is no big deal.
- oh I'll have to do that at some time
- Yeah, the everyday usage is limited to, well, everyday sizes.
  As you pointed out there is no difference in everyday usage. But for anything outside of the most trivial of comparisons, the imperial system breaks down hard.
  The advantage of the metric system is the possibility to scale both bigger and smaller when you need it, and always does so with a consistent factor. Sure, not needed when you want to know how many 8 cm strips are needed to cover 50cm. But for 1m? With inches and yards you already have to handle two conversation factors. How many sixteenth of an inch do you need to cover a foot? 192. Possible to calculate, but not nice and you'll approximate with 200 if you need to do some calculations in your head. How many feet are in a mile? 5280. And yards? 1760. Do you really know these conversation factors? Do you want to calculate "there's a street light every 30 yards, the street is 2 miles long, that's xx streetlights? Or "there's a street light every 30 meters, the street is 3.2km long, that's 3200/30 = 107 streetlights"?
  Oh, and the weird units do actually get used when it is a convenient size.
  Cooking uses lots of in-between units for example. Centiliters (cl) are common in cocktail recipes or for shots, in some cookbooks you will find dekagrams, etc.
  Hectare is commonly used to give area measurements (it's origin is hekto-are, and "are" in turn is hekto-square meters, though "are" is not commonly used.). Want to convert hectare to square kilometers? Simply divide the number by 100! 3000hectare of forest burned down? That's 30km^2, so 5kmx6km. Easy to visualize. The US customary system makes such conversations really really really hard. How many square feet, yards or miles are in an acre? I just looked at the Wikipedia page and there is no way anyone will be able to convert an area given in acre into "well, it's approximately x by z miles". Or "my house have xx square foot of living space, so that's yy acres".
  Btw, no one uses kiloliters, because that's equivalent to cubic meters. Easy conversions!

Here in Sweden and Norway we have the Scandinavian mile, "mil", it is 10km.
- We also use deciliter and centiliters, which is odd for some.

In our primary schools, we learn our children mili, deci, centi, deca, hecto and kilo, and how to calculate between them.
Beyond that or below that is used either in science classes or specific usecases and not known by the whole population at large.
Since people use what they know, they'd never use mega as a common way of measuring. We mostly use km for distance, and only in specific cases we might use, say, hectometers or decameters.
5 megameter is not wrong, but I don't call 34 cm 3,4 decimeters either(unless decimeters make sense of course :p)
- Here i learn mega, giga, tera. But show up in computer which is like place you most commonly see them.

It is 2.445 kilomiles from Los Angeles to New York.
- that's a whole 12 Mft
- 38.73 megahands

Not much need to use Mm, it doesn't come up very often. So when it does it's easier to use thousands of km so as to not confuse people with "another" measurement.
- We routinely count vehicle odometers in thousands of kilometers, AKA Megameters. I'd say it's a common enough measurement to popularize Megameters
  
  That really doesn't come up often enough.

Yeah but the real question is how many is a Brazilian?

Megamind?!

How many petameters is that?
- This sounds like some electronic device they'd roll out in the UK used to detect pedophiles.
  
  No need to worry, it us just your everyday AI system

I've seen megameters used in the context of astronomical distances, but not terrestrial ones. I think terrestrially, the familiarity of kilometers helps with a sense of scale.
- The beauty of the metric system is you don't lose a sense of scale from using a higher unit because you can intuitively know 1Mm is 1000km.