It's done that way as an overreaction to B2B customers which may want totally isolated per-tenant systems.

Take Okta login for example. Okta wants to offer big hyper-secure customers an option of "if you want, we can run our system in your cloud/data-center/whatever". To support that kind of system, you go to to the https://login.okta.com/ page and enter your email, JUST your email. Okta uses that to look up which customer tenant you belong to, then sends you to customer.okta.com where you enter your password. This way, the password only goes through infra owned by big-customer.

Okta then just builds everything with his indirection so they can move customers to it.

There is no pair for the enterprise users signing in with their company's SSO or those using Passkey.

I think what some sites do is have a visually hidden, not required password field that a password manager can fill in. If it's not a password-based auth, the flow goes to the next step but if it is, it reveals the password field which may already be filled in.