On a similar note, why all websites have login and signup forms, with the registration typically on the home page and a (huge) fallback logic to log users in ev

On a similar note, why all websites have login and signup forms, with the registration typically on the home page and a (huge) fallback logic to log users in even if they use the signup form? Plus the fb/google authentication buttons with the same logic twice. Imo it’s just because “everybody else/the bigs are doing so”, and nobody cares to test a different thing. Note that this isn’t trivial as it seems, as you may have the fb/google buttons, but the bigs don’t.