SO questions aren't really intended as a messaging or conversation forum - they are closer to a wiki of questions. Wikipedia also doesn't allow pleasantries or signatures in articles either.

That's the crux of it, though. What conversation?

Stack Overflow was created specifically to avoid conversation. Hence the treatment of comments, and the fact the posts on the page consist of one question and everything else being a candidate answer (and not part of a help-desk or troubleshooting process). It was created by people expressing a common frustration with discussion forums, where you scan ungodly long threads without even knowing whether an answer got posted, have to ignore back-and-forths that are completely irrelevant to you as a knowledge-seeker, and perhaps ultimately end up at a "never mind, solved it" post with no elaboration.