1) Uptime (though this could be partially alleviated by retries)
and most of all:
2) "Trust"/"Spam score"
It's the main reason to use Sendgrid, AWS, Google, etc. Their "value" is not the email service, it's that their SMTP servers are trusted.
If tomorrow I can just send from localhost instead of going through Google it's fine for me, but in reality, my emails won't arrive due to these filters.
See jwz's struggles with hosting his own email. (Not linking to his blog here with HN as the referrer...)
With email, the 800 lb gorillas won, and in the end it didn't even solve the spam problem.
I have a 15+ year old Gmail account that I've used everywhere. Spam has been a non issue since 15+ years ago.