Fastmail and protonmail offer a lot more than just postfix+dovecot hosting. Fastmail developed new standard, brand new webmail UI, brand new webmail client. That's a lot of added value. Protonmail is heaven for crypto nerds which has its own intrinsic value as well.

Ordinary mail hosting is nothing special. Anybody can do it with very little investments. Of course if they can charge for it, good for them. But I don't think it's fair to compare those services. It's like comparing AWS and some random VPS hosting with cpanel just because both can launch debian VPS.