I see your point, and there are some open source projects aiming to do that (for example Zulip), however it seems to be incompatible with the goal of a Q&A site, which is what we want to build here.
Ultimately a Q&A site is about (as the name says) questions and answers. There may be comments and votes and that kind of things, but they are all secondary in comparison to questions and answers.
We had some similar ideas for comments, though. They are supposed to be threaded. See more in this discussion: