They don't, though[1]. It's often the lowest-common-denominator because everyone has it. But if a more streamlined alternative, like Slack, is available then it will be preferred over email.

A quick look at websites talking about email trends finds that the only people who are bullish on email are marketers. And that's why everyone else hates it: it's predominantly spam. So if you want to compare ActivityPub to email you'd better have a strong argument for why it won't just become another vehicle for spam if it ever becomes widespread.

[1] https://www.cnbc.com/2018/05/16/personal-email-is-dead--but-...