I think we do that because what happens if random contributors jump in and start adding even more random task lists? We could probably do some checks to prevent that, but this is at least a safe and practical way to do it as a first-pass.

Yeah, I can definitely see that for public projects. That said, the subsequent comments could just be edited to remove the extra checklist if it were an issue.

A similar argument can be made the other way: The OP can be edited to include further tasklists if needs be.

True, but then you lose comment notifications, and edits can't be made simultaneously to the same post by multiple users.

You can get comment notifications by hitting the Subscribe button. The latter seems like a good problem to have, and one which you would normally not hit if you're still using Github issues.