I can’t think of any good reason why we would need 2 buttons to log in.
If using social login, it doesn’t matter at all because from a user perspective it is equivalent to login or sign up. It’s a click of a button.
If using user&pass then and only then you can offer to open a new account…
The reason it’s frustrating is that currently if someone is not registered and clicks on “Log In” and chooses some social login (Google, Facebook, etc.), then they are redirected back to the Log In screen instead of the “choose a username” screen as would make sense to do. The user should have known to click on “Sign Up” but what if he didn’t remember whether he signed up before? what if he just made a mistake? I think this confusion is rooted in that there are two buttons where we should only have one. If the user is not registered, then seamlessly register them.