Someone liked the idea of writing a game engine in Rust and so they did. I wrote my own sidescroller engine when I was 16 as well, although there were existing things around and it thought me a great deal about how to tackle multiple classes of problems.

Now the bevy devs seem way more experienced and if there was already a game engine that ticked all their boxes they might not have started developing a new one. But the point here is: Not everything is a product with an clearcut target audience — and not everything has to be.

Projects like these ultimately help the whole field of game engines to move on and innovate.