Bad code can have a really high cognitive load to maintain. Just to illustrate what bad means, take a look at this:
https://www.reddit.com/r/programminghorror/comments/1cb6rca/...
I can see that having even bits of modification impacts lots of lines, where if written just a bit more saner, it could impact just a few.
Of course everyone is absolutely free to do the things however they want on their project. It's just that bad code, and bad choices bite back sometimes really badly. Project Zomboid is built in LUA for example, and it shows, it's a horrorshow not just to play, but to develop it as well. Their programmers spend a lot of time with just refactoring things. Besides functionality, maintainability should be a huge focus in my experience, so that the devs don't hate life if they have to touch the code again.