Some games really can be paid. Some games can be with no pay. So it is decision of creators. In theory developers choose it on their own.
But, in a real way, some people just has lots of greed!! So casual answer - GREED! They set a price, cause of GREED! And nothing else. Maybe they can think they are so cool to have a game with a price. Maybe.
But, you know, any dev, one day, will try this point. To set a price for a game. And try to earn something. So you are try to get some result. You invest into dev lots of time. And you want something back.Some positive. For example try to do a game, you can do. Make it better. And set a price. This is also a way of self progress.
As for me, i am thinking about to set a small price for one game, i am doing right now. Because of i invest into it lots of time.
So i can do just a game, i can do. And say, ok, i invest X time, and this is enough. I have trained in programming, i have trained in doing art, i make idea i wanted. And i dont care about anything else. It is my way, i see it this way. And thats enough.
And there is situation - you invest more time, that to just do a thing you wanna. You try to plish game. To add more pixel art, to do more code. To add more features. That are far beyoud, just do the game you wanna. You try make it better. You invest lots of resources into this idea - make it better, than just a game you want to do, than a game for training. And at this point you can try set a price.
I also have ideas about to do a release a pack of games. And set again possible price. So pack of games. And you can transfer some money, or download for free. There is such an option too. But this is about a pack of my basic pascal videogames. You can dowload pack for free. Youc an download every game one by one from websiute fo free. Or you can set some trasfer on money for a pack, if you wish. This is idea also.
So, i am thinking about such things, somethimes.