Because software engineering stopped being rational and has largely become a game of emotions and marketing? If you can trust Google and Facebook, then you can trust Microsoft.
Google does not have the same history of hostility towards Open Source and the market in general. Microsoft continues to be adversarial to almost all players in most markets, so why would one expect them to be different here? Only those who have short memories and are not students of history.
No one currently trusts either of those two with their projects, and very little in tech actually trust them with private information, their main source of revenue.