I used to host my own mail server. Getting it up and running with iredmail wasn’t too difficult, but maintaining all of the different components and setting up spam filters and autodiscover and stuff like that is an absolute nightmare.
I just use proton mail. I can point my dns to them, and they do everything else for me.
Only downside is that they don’t expose pop3 or imap, so you have to either use their app, or set up their bridge and host that locally.
It would be like saying that if 99% of Linux users used RedHat.
AOSP is open source sure, but realistically basically everyone is using a closed source version that the OEM has messed with.