Because self-hosting a mail service is totally something my mom can do...
(As the sibling comment notes, I'm talking from the perspective of the end user, not that of a technologist. We need to remember who we're ultimately building these things for...)
So your theoretical (straw man) user cares enough to not want their email in the hands of a third party service provider, but is unwilling to run their own service? Maybe this user should deliver their messages by hand directly to the recipient, because their demands are unrealistic.
Self-hosted services are a significant step forward from the corporate trap of the 2000s. Offering paid hosting plans is the most logical, sustainable revenue source for the companies writing this software.
No, I consider anything where step N involves "go to Github..." a non-starter for anyone who's not already a technologist. Self-hosted is great — in the world Snowden has demonstrated we live in, it's probably the best solution — but it needs to be turn-key, "plug it in and enter a username/password and you're done" level of effort, or it will never become widely used.
I imagine statements like yours are why Google has the user base it does.
[Sscene. PM_Tech has phoned his truck driver father]
"*Yes Dad; just build a simple installer to get an email address. An email address...it's like a phone number but letters instead. You know...so the cable company can message you. Well I suppose you could just call them...I don't know, it might take a while to build. You will have to learn programming for a start. I know you are 56. Yes, it will cut into your time for watching sports. I know I am "good" at that "computer stuff" but some guy on HN said this is how you should get an email because he knows how to do it...OK. I will be round for the game. Love to mum."
FYI Any service I have to use that starts with going to GitHUb will be ignored unless
[a] I am trying to learn something from it
[b] It is 2048.
Your mum likely doesn't have the need to connect multiple email endpoints through a single API either , your mum is not the intended audience of this product as it stands.
(As the sibling comment notes, I'm talking from the perspective of the end user, not that of a technologist. We need to remember who we're ultimately building these things for...)