My favorite is it's often paired with "passwordless" trash lol.
Why can't I just give my whole fucking credential out in 1 action. What's this nonsense where I have to enter my username, THEN wait for the page to load, THEN click "send verification email" or "send code", then half the time they want to SMS me and have me enter another code lmao.
I can’t speak to all of them, but many sites that require (only) a username first have enterprise SSO integrations.
The enterprise buying the service (understandably) doesn’t want its employees to type in both username and password on a 3rd party site, especially since the SSO process will handle auth after the username is entered.
I know of one site with both username and password on the first page, and has enterprise SSO. Login will automatically fail if you enter anything in the password field when logging in with an SSO-enabled username. But that doesn’t stop copy-pasted credentials from being transmitted to their server, which is something enterprise customers want to avoid