Apart from the "users lose their passwords all the fricking time" problem (seriously, before we implemented
https://fastmail.blog/2017/12/06/security-account-recovery/, lost password was always in the top 3 most common support requests of the week report)
Impementing per-message-encryption would turn us into a dumb blob store. The whole point of FastMail is the value add - fast search, ability to deal with a lot of email quickly, etc.
That and people's devices are basically always on these days, and fetch new email immediately on a push when messages arrive. So if your provider get a subpoena or gets hacked, then a push request will make your device connect with the password, and boom - access granted.
Finally, we don't let people store master passwords on their devices any more, because they get leaked due to hacked devices, so we require people to create app passwords. This would be in direct opposition to many of the other safety things that are done.
(extra finally: phishing protections and antispam solutions are in pretty much direct opposition to the idea of the server not being able to see the content of emails)