Well, to put it simply there are these things called microtransactions, basically you want items in a game or extra lives or something like that, you can pay for them instead of earning them, sometimes they make it so that certain items can only be paid for, worse they make it so that certain items can only be paid for and will only be offered for a limited amount of time. If you miss the window to buy them now you will never be given another chance. Normally this is something cool like a tie in with a new movie that came out or something of that nature. Fortnite does this a lot, hope you got those Marvel characters when they were offering them cuz you're not getting them now.
But as if that wasn't bad enough there was another layer to it, one of the things you can buy with microtransactions, using real money, is a form of money that can only be used in the game.
So, what you give them a dollar, they give you 100 coins, and there isn't even exchange rate? Of course not
There are various bundles where you can buy the premium currency as it is often called. Typically the more expensive bundles give more, and it's not tiered properly, so let's say $5 gives you 800 coins, but $10 gives you 2,000 coins, it's to goad you and to always buying the higher amount, even if you only want that one item.
But it can get worse, they can set the prices so that you can just barely afford the item you want with that $10 tier, so the next year is 5000 coins for $20. And with that you can get enough coins to buy the item you want and have just a little left over, but not enough for you to do anything with unless you buy a lot of coins to supplement that amount, which can trick you into thinking that you're getting a good deal when you are actually being fleeced pretty hard.
Fortnite is so bad because despite it being a good game, it does all of the above and targets to children who don't know anything about money.
There are cases where you can buy one form of Premium currency with real money, so that you can buy a higher tier of Premium currency with the premium currency you bought with real money, forcing you to pay even more.
And this is one reason why modern games suck, the other reason is that everyone is using the same Engine.