There is a big cost of context switch. If you are such a genious, that you can just jump around in entirely different codebases fixing entirely different set of problems, in a way that actually improves things, than hats of to you, because I can't. I am not bored and I am fully occupied maintaining my own OSS codebase and I doubt some random dude could just come and fix a problem there (unless it is a really trivial one).