Also "sign up" has replaced "register" and "create account".
Does anybody know why?
When you go into a physical location, say visiting a hospital or business, you "sign in" and "sign out" of that place.
When you participate in an activity, for example a class or sports club, you "sign up" to join.
Sign up and sign in confused some people because of how it mentally translated to some people's native language. We targeted more elderly people too, who didn't know the difference.
I assume that log in and log out might translate poorly to some other languages. Lots of people optimize heavily on registration, like even a 1% difference can be significant. And copy is low effort, so I assume there's a reason it evolved this way. Maybe it's a hint on which market the website targets.
Companies optimize for getting new users. Highlighting the register button reduces some friction that may have otherwise prevented some users from signing up.
On the other hand, a user that wants to sign in is much less likely to "give up" if they don't immediately see the button. Important and limited header space therefore is dedicated to getting new users.