Why do most people stay in centralized social media platforms when the Fediverse exists?

Network effect. The people they care about are already in those places. Their family, grandma, a distant part of the family on the other side of the world, their favorite celebrities, people they went to college with, etc.
They don't know about the fediverse.
They don't care about privacy or things like an open web or corporate control. They "have nothing to hide" and think it's pointless to resist corporate enclosure of the internet plus the centralized platforms are free and store their photos for them and take only things they can't see.

What makes a social media platform good is, well, socializing, so people are naturally going to flock to where the most people already are. I am only here because I am blocked from r*ddit.
- I have been known by so many users names in Reddit. You can just create a new account.
  
  No I cannot. I am blocked. Not banned.

because it's obtuse, i don't even understand it

From a purely practical standpoint, big platforms have (potential) reach that small ones simply don't. Sometimes the censorship is so bad it's hard to make use of that reach, depending on what message you're trying to do, but nevertheless, it is a factor.

Because almost none of my friends, family, and neighbors are on there. If I want to stay in touch with people and keep my organizing relevant I need to keep accounts on the most popular social media sites up.

I have a background in programming and UX. The fediverse is just horrible and confusing to use.
For example, take Lemmy itself. Some instances are federated, some are not, some instances ban other federated instances, etc. on Reddit, I can just go to r/piracy. On Lemmy, I want to go to an equivalent.
I look up “piracy.” Okay, there are 10 options. But the one I want to go to (piracy@dbzer0) is not on the list, even though I know it exists, because its instance was banned by the instance I have an account on. And even then, depending on the instance, you might be able to see and subscribe to a community, but the last posts visible to you will be from 6 months ago because the newest posts are not federating to your instance, and there’s no way to know or fix this. At this point, the average user would say “this piece of shit website doesn’t work, I don’t want to use it” and leave.
The truth is, the average user wouldn’t even get to this point. They want to use “Lemmy,” then don’t want to use or understand how to use an “instance;” like I said, they want to use “Lemmy” not an “instance.”
Lemmy fucked it up from the beginning for having instances that can create their own communities at all. They should have had a central URL (like lemmy.world for example), and then let people host communities only. That way, all of them would have been connected to a “Lemmy” service, while still being able to control their own community’s data.
At least on Mastodon, you can ignore the federation aspect almost completely. But the truth is, the Fediverse is just extremely poorly designed, and it will never get mass adoption unless fundamental changes are made.
- For real. If you're not inclined to join either the leftist or the tech instances, there's not much point being here. It's just a clunkier version of Reddit with fewer people and even fewer original posts.
  It doesn't even do anything different from Reddit even though the opportunity to set itself apart is there. I mean, lemmygrad and hexbear constantly post new megathreads for casual discussion so why not implement a chat feature directly onto the platform? Why can't we follow a post? Why can't we create different subscription boxes? (I'm pretty sure Reddit lets you do that actually)
  idk it's just lame that we're apparently content with being reddit jr.
  
  100%. It’s even more annoying when tech nerds in charge refuse to improve the experience, wanting to keep their little bubble of superiority closed off. Then they whine about how nobody wants to use their perfect solutions and prefers centralized commercial offers