Pretty simple root cause: Bureaucracy combined with lack of shared vision.
I support your need to vent, it's undisputably a magnificently huge pile of stinky garbage. Given the amount of resources poured in, it'd be hard to do worse.
What's particularly sad is that they're just cloning an existing thing. The "vision" need be nothing more than "see this, make something exactly like it that works". Google made similar mistakes with Google Chat but thankfully nobody uses that.
Nah, Google's mistakes with Chat are myriad but the biggest one is that they didn't make Slack. They added like one feature on top of Hangouts (threaded group chats), completely redesigned the entire interface (and the Gmail and Meet interfaces while they were at it), changed a bunch of words to make it seem like they copied Slack, and then said "enjoy using this new turd we pushed out".
(Okay, they also added a tiny bit of Markdown-based text formatting. Because why not use the least intuitive, most complicated possible solution, and then not document half of it.)
I support your need to vent, it's undisputably a magnificently huge pile of stinky garbage. Given the amount of resources poured in, it'd be hard to do worse.