Back in like 2004 or 2005 you had a BlackBerry. There was an “app” on there (they weren’t called apps back then) where you would see all your incoming messages. Any type of messages, doesn’t matter what “app”, what service, it was all there. For example:
Incoming email from gmail
Incoming email from Exchange
Incoming and missed phone calls
You get the idea. It was all there in one pane of glass.
I got my first Blackberry (issued by my employer) in 2004 and was immediately addicted1 to that unified messaging paradigm. Nothing else has come close to copying it. I suspect that for most people the stream from all of their services in one place would be completely overwhelming. Just triaging my email in one app is bad enough. Compounding it with text messages, Slack, phone calls, FB/Twitter/whatever would be unmanagable in 2021.
1When I had to turn my device in at the end of the job, I immediately drove to the nearest Cingular store to replace my personal device with a Blackberry; I could’t be without one.