Answer: Because the username/password isn't the only way to login.
If we want to support both SSO and username/password login, the first thing we need to do is to check which way the user has been using.
For example, if the user is using the Google SSO, then you check the email, make sure he/she is using the Google SSO, then show the Google SSO option.
For someone who thinks the username/password is the best way to login, it's not surprising that they feel cumbersome and "user-unfriendly" for splitting the username and password into two pages.
Still, if you want a single page to handle both SSO and username/password login, based on this article, there are two options:
- Show SSO options after the user types the email
- Optional password field