This point cannot be underemphasized. "Thinking aloud", not just in front of another person but under a very stressful situation (for most people) that also basically almost never occurs otherwise in daily life -- is a specific metacognitive skill that for many people can only be (even adequately) learned by either (1) careful training or (2) surviving many repeated failures, and finally getting at least partially acclimated to these kinds of confrontations.
Yeah sure, perhaps some of the people who "bombed" that calendar test really were the walking fraudsters the interviewer (who knows they'll get to stay in their well-paying job regardless of the outcome of said interview) makes them out to be. But most are probably simply nervous, and due to a variety of psychological factors (imposter syndrome, among others) are simply momentarily blanking out, and having suffering from a very common form of mild anxiety attack which makes the problem seem much more complex to them (at that moment) than it otherwise would, under more natural circumstances.