In theory because the code being added is introducing a feature so compelling that it is worth it. In practice, that’s rarely the case.

My personal approach to open source is more or less that when I need a piece of software to exist that does not and there is no good reason to keep it private, it becomes open source. I don’t do it for fun, I do it because I need it and might as well share it. If someone sends me a patch that enhances my use case, I will work with them to incorporate it. If they send me a patch that only benefits them it becomes a calculus of how much effort would it take for me to review it. If the effort is high, my advice is to fork the project or make it easier for me to review. Granted I don’t maintain huge or vital projects, but that’s precisely why: I don’t need yet another programming language or runtime to exist and I wouldn’t want to work on one for fun.