Mostly because it’s just easier to let someone else maintain an email server than to do it yourself, isn’t it? Theoretically there’s nothing stopping you from spinning up your own email server at your own domain, but it relies on DNS registrations and there’s little incentive to do it individually. Someone correct me if I’m wrong.

Hopefully paid relays will fix the incentive aspect for nostr. Also, who knows? I’m feeling a little inspired to run my own email server all of a sudden. Maybe nostr will inspire a resurgence now that it’s a lot easier to host one’s own server these days.