No evidence - how would you even begin to get it. Just my feeling from doing many interviews as well. I still recall a CS PhD from a top school fumbling with the most basic for loop in an interview I was doing. I just can't believe people are this incompetent. I think the simplest explanation is nerves - even mild levels of stress are known to shut down people's ability to think.

Ergo, I strongly suspect the current tech hiring culture filters out anyone who has above average sensitivity.