Some major sites like Google and Amazon take 2-page approach for signin. They first ask for ID and in the next page ask for password. What is the advantage in this against single page signin process? Is this because of security, technical, usability or any other reason?
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.