Login fatigue. Yes, I use a password manager (1password). But it means I need to stop going through the sign-up flow, jump over to the password manager, create a new login, change the password recipe from random-letter to random-word (which I prefer), generate a password, then go further modify it to accommodate the site's particular password complexity rules (upper/lower/special/numbers), add the URL, save, then go back to the sign-up flow to finish.

And... 90% of the time, that's what I do, because I prefer to have separate logins for every site.

But sometimes, I just click the 'sign in with google' because it's a hell of a lot faster and easier.