Guessing here, but probably because it's extremely difficult in the modern day to host your own email server; you'd be depending on one of the big/established players to handle the traffic.

I've run my own email since ~2013 or so, and if I had to set it up from scratch today, I probably wouldn't. There's just so many hoops to jump through to get it going, secure it properly, and get your messages accepted by the big players. Not to mention, getting a clean IPv4 address from a VPS provider that isn't on every spam block list is damn near impossible. With some work, you can clean those up, but Microsoft has some internal block lists that are extremely difficult to get removed from (there's a form you can fill out, but good luck finding it). I'd guess other big players have similar "internal" block lists. With those, the email doesn't bounce or give any indication there was a delivery failure, it just silently goes into a black hole: the sending server assumes it's accepted, and the recipient never sees it.

Also, most residential connections flat-out block outgoing SMTP on port 25, so self-hosting would not be possible for many.

If an email-based chat/IM platform were to eschew compatibility with existing email services and just used the protocols on different ports, it may be feasible, but it would have to run in parallel to other email services or rely on some kind of "bridge" setup (probably not what those projects are going for).