there are plenty of tools to take code from games, can't you use those instead or is there some legal stuff i'm missing?
Decompiling is not the same as providing source code. When source code is compiled into a binary you lose eg the names of functions and variables, and there are tons of different ways to decompile a binary executable as there are always multiple ways of writing code that compiles into the same executable code. Decompiled code requires a lot of human attention to make it readable, maintainable, and easy to work with. You would not be able to get Maddy's original code from a decompiled Celeste as there will be so many places where you don't know what she named a variable, or how exactly she coded something. Just because you have the binary executable of a game doesn't mean that you know how the source code was written. Releasing the source code would tell us exactly how the game was actually written.