My understanding is that no one has built a slack like client for matrix. Whereas at least keybase has something that looks like slack with teams, channels, and threads. But I've only played with it in a sandbox.
I mean there's lots of open source stuff. Signal is open source and people still complain about it being centralized but just won't run their own servers and connect them together. So I think open source isn't enough. It takes people working on things.