Looking at it from the candidate's point of view: Being rejected is actually something that psychologically kind of sucks a lot. Even if at the level of higher cognition you are perfectly aware that it's just a numbers game, and that you shouldn't let rejection get to you in a psychological way, we are not vulcans but humans. Rejection means cognitive dissonance in a big way: You applied for the job, so that means you wanted it. But they rejected you, so that means that you're not getting it. It's also a threat to your identity, because you think of yourself as being pretty good, and now there's someone who thinks you're not good enough for them. It just sucks.
So what do you do? You engage in cognitive dissonance reduction. You look at that recruiter spam, and you immediately start looking for reasons not to apply. Because if you find any reason not to want the job, then they can't hurt you by not giving it to you.
Recruiting is broken in a big way: We need to find ways of doing it that causes much less psychological friction.