No one stops you from running your own email server, it's the pain to deal with that's why people pay someone for the trouble. YES you can run it, no one said its easy. Google, outlook, etc does it because they are better than average joe in managing it, plus they have incentive when they sell your data... You do have a choice, the question is whether its worth the hustle or not. Nostr is too young to judge, it remains to be seen. It does have huge potential but knowing people, someone will screw this one up for everyone..lol