My guess is that the cybersecurity folks have more say in that case than the UX people.
Transitioning from click to keyboard to mouse takes a lot of mental energy and doing so multiple times separated by a page reload is a good argument against that practice from a UX standpoint.
However, I would guess that companies like Amazon and Google are doing a lot of "work" behind the scenes during that screen refresh. I would be checking to see if this is a valid email address. If the device location seems normal. And so on. All while blocking form fill scripts and brute force user/pass combo attacks.
Since the sites in your example are big players with a lot to lose from a security perspective, and because users of those sites want to access making them willing to endure a bit of pain, UX kind of loses the business case.