Why separate login and signup page?

I was wondering why basically all webpages use separate login and signup pages. Not necessarily separate html files, but they are usually built as two separate forms.

Why not have one form with email and password fields and two separate buttons, one for logging in and one for creating a new account? The only two arguments I could think of against this are

  1. People are used to separate pages.
  2. You might need additional data for signups.

The first one isn't really a strong argument because if the page is built right people will notice the difference right away. The second one also doesn't count for much in my opinion, because a lot of pages actually only need the email address, and also a lot of pages ask for additional information on a separate page later on anyways.

Do I miss anything?