The reason why they do this is because of sso logins for other organizations.
For example, to log into my college gmail account, I type my email, get redirected to the schools sso page (retype my username and password), then get redirected to gmail.
For example, to log into my college gmail account, I type my email, get redirected to the schools sso page (retype my username and password), then get redirected to gmail.
That's even worse than the non-sso sign in flow. It definitely doesn't have to be that bad.
At the place I work we use office 365. Their sso path is so much better. I never need to re-enter my username.