Because email runs on protocols, formats and implementations built decades ago and running everywhere. It's why it's so universal and reliable but also why new advancements can't really happen - unless everyone is on the same service using proprietary features.
I’m certain that advance can happen. Just look at Calendar invites. They certainly weren’t envisioned at the time that email, MIME, SMTP, etc were started. But calendar invitation were cleverly designed to work within email itself. I know many users who don’t even realize that when they create calendar events and invite colleagues at other companies to a meeting, that that is actually just going over email. The clients do a good job of obfuscating that fact.
