I don't think there's generally a cultural reason. It just doesn't happen all that often because the "someone else" coming along probably isn't familiar with the project workflow either so they don't know the right way to do it.
What does happen more often I think is that one of the maintainers or regular devs will make minor fixups as part of accepting the patch. (I do this for the project I work on for new contributions at least some of the time.) But that does take time, and usually the people working regularly on the project have a full todo list of stuff they care about or are paid to care about already. So if the patch is for an obscure and unloved area of the codebase (like, say, support for a networking protocol used only by ancient Macs) it's more likely to fall through the cracks simply because nobody working regularly on the project cares about that area. Niche use-cases which have users but no developers are fundamentally in trouble long-term, I think.
What does happen more often I think is that one of the maintainers or regular devs will make minor fixups as part of accepting the patch. (I do this for the project I work on for new contributions at least some of the time.) But that does take time, and usually the people working regularly on the project have a full todo list of stuff they care about or are paid to care about already. So if the patch is for an obscure and unloved area of the codebase (like, say, support for a networking protocol used only by ancient Macs) it's more likely to fall through the cracks simply because nobody working regularly on the project cares about that area. Niche use-cases which have users but no developers are fundamentally in trouble long-term, I think.