What prevents the hacker from cloning the whole web-page of, say, facebook.com login and phish users for credentials this way? This is not a hyphotetical thing, Kali Linux even bundles a utility program for that.

Compared to that, one icon, that is the same as that of the company, is not that threatening.

Not only that, but if you consider a sign in form that wouldn't have a logo, it would be way easier to trick user into putting their credentials in, because the user wouldn't be able to differentiate them. Also OAuth is always branded AFAIK.

Users may also notice discrepancies in the logo, if it was cloned poorly. Though I can't think of a way someone couldn't forge a logo given all the possibilities. Adobe Illustrator can trace images into svg and there's plenty of companies' svg logos just in the google search.