True enough

Well it‘s true that one can use RSA, which is still save with keys big enough, but if someone wants so save some extra computing power and time ed25519 is the way to go.

Learning by doing is the main example most people will selfhost things. For example why would you want to solve a puzzle if you can buy it ready made? Why building a statue of lego bricks and not buying the built statue? Because learning about something you like is fun and you gain experience from that which might be beneficial for your present or future job.

Also some consider data privacy more than others. With selfhosting you own your data and not the company behind it. Why should I make an account for a recipe app if I can selfhost it and not needing an account? Why sharing my private information with some random tech company?

There are a lot of things people want to accomplish by selfhosting.