Github would like a word.

If the self-hosted email infrastructure goes down, roughly one company is deeply affected. When Microsoft's server goes, entire industries and governments will simultaneously be unable to conduct business as usual.

From the individual's perspective Microsoft might seem like the better choice if they can achieve higher availability, but the societal cost is so much greater when the one basket you've put all your eggs in breaks even if everyone holding their own egg on average results in more broken eggs per year.