Also once you move from college into the working world you have a work email separate from home email typically. I check my work email daily of course since it’s critical to my job, but I can go days without checking personal emails.
Most of the content of my personal email is hardly time sensitive and a lot of it is advertising or news I haven’t opted out of (although I have gmail filters and tabs). It’s not like IM where I would use to check and respond immediately because I only ever got messages from friends.
Slack and GitHub together are adequate for day-to-day work things. Everything else just isn't sufficiently important to justify a channel into my brain.
The problem with email is it’s too easy to send, and I get 100+ non-spam per day between work and personal. I do not have time to read, let alone respond to 100+ emails per day.
So I triage them as best I can, and a lot get binned based on sender or subject alone.
The walled gardens improve on email and XMPP because they’re walled.