[rwalle]

There is an obvious reason.

Facebook and Google provide "Sign-in with Facebook/Google account" not because they do it out of goodwill, to only make it "easier" or "smoother" to login -- it obviously cost resources on their end to enable such features -- it helps them better identify users and then serve ads. And Google can be really aggressive -- try reddit or Quora.

Apple, on the other hand, tries to sell "login with Apple account" with a different approach: they advertise the "privacy" part of it and how you can hide your email address by using it's sign-in service. And they have a term where login with Apple must be enabled on an app and website if a company has an app on the app store and it supports any other third-party login. In other words, if Reddit supports login with Google on iPhone, it must also support login with Apple ID. This helped the adoption a lot.

For Microsoft, they are relatively late and small in the ad business (for now) so I guess they don't really care about getting more of your information via sign-in services. And they are not on this privacy bandwagon as Apple does. So they really have no incentive for this.