in most cases you lose a lot of the information in the source code during the compilation process, making decompilation significantly more difficult.

there are cases, particularly with newer games that are less "close to the metal", where decompilation works really well (hell, it's the reason why minecraft modding is as it is), but for older or more demanding games those kinds of abstractions would often result in less performance, meaning the devs would spend more effort squeezing as much performance out as possible, which was often done by programming in a lower level than games of today