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.
Even if your "self hosting" is renting a $5/month VPS, some spam lists (e.g. UCEPROTECT) proactively mark any IP ranges owned by consumer ISPs and VPS hosting as potential spam. I figured paying fastmail $30/yr was worth never having to worry about it.