The alternative is to let the user choose the authentication method right on the first screen, but that would mean showing all possible methods (some of which may be limited to certain groups of accounts) and getting a lot of users stuck because they chose the wrong method.
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.
Plus if you have a bunch of tabs open (G Suite), it will log you in all of them once you log in on one.
Oh god... How a human being can stand this humiliation? I image this inflating to eight steps in 2030: login, password, 2FA key, face scan, reCAPTCHA, ID scan, phone call, click checkbox in AI TOS no one reads, final click on “We’re done!”