Also, many are simply not comfortable with the situation and too stressed to be engaged in any kind of intellectual activity.
That's not to mention that most cannot do thinking and verbally document the process at the same time. It's either or.
But also on a fucking whiteboard! You're thinking about your handwriting, trying to form shapes you've probably never written (like curly brackets). You can't copy/paste and move things around when you notice you made a mistake. You have to remember syntax and stdlib APIs, which is not everyone's strong suit (especially in languages with inconsistent naming like Python and PHP).
It's just totally insane to say that whiteboard coding is anything at all like working as a coder.
The main challenge is that you have to write what may be incorrect in order to get to what is correct in most problem solving. When you have people watching and potentially judging you, you try to program in your mind which is difficult. Then it shakes you because time is passing and you haven't written any code. They're drinking beverages and you're sweating, haha.
I will say however, that the company that had me do a whiteboard was very professional in how they handled disarming all these things you mentioned from my mind when they sensed them.
I got offered a job but I had several other offers. But I still had the sense that they were a good company.
I don't think whiteboards are fair, but I also don't think it's fair to write a company off for using them in their interview process.
If you're going to do them, inform the candidate ahead of time and also help put them at ease when you sense the inevitable anxiety. Let them know that you really want to see the approach not the end result and writing bugs is ok.