That's not been my experience - I've successfully self-hosted email for nearly a decade now.
Really nice guide here: https://thomas-leister.de/en/mailserver-debian-stretch/
https://cfenollosa.com/blog/after-self-hosting-my-email-for-...
It's impossible to self host with the guarantee most emails sent from that host won't be outright rejected or flagged as spams by centralised mail services.
IP/domain reputation is also probably an issue; I’ve been using the same IP address and domain names for over a decade so services like Talos are reasonably satisfied I’m harmless. If someone sets up a server and it has never sent mail for their domain before—or, worse, they get a radioactive IP address—it may be hard to become accepted by other email providers.
I get the sense that the relative lack of trouble I have maintaining a mail server is more to do with being effectively grandfathered in and worry that if I ever have to shift away from my current provider that email sending is going to become a lot harder since I won’t have my IP reputation any more.
Not that there are zero issues, in particular if you set up a brand new mail server with zero reputation, it can take a while to get yourself out of the hole.
I think for normal people though it makes a lot more sense to just pay for email. I'm not sympathetic at all to someone who complains about the quality of service of free Gmail. You are the product, so for the ten millionth time, it's going to suck for you somehow. Fastmail costs 3 bucks a month, if you have 3 bucks, go buy it, it's better (or buy one of their many decent competitors).