There are many possible reasons. Could be due to lack of effort, shyness, ugliness.
I am one of those guys. I am in my late 20s. No woman has ever been attracted to me. It isn't hard to understand why - I'm ugly, 5ft3, and have a minor physical disability. I've been called "ugly" to my face and behind my back before (by both men and women), and women have made comments such as "I'm too short to ever find a partner".
I do reasonably OK in other respects. I have friends, a good career etc. But I'm simply not attractive to women. And that's fine - people can't exactly force themselves to be attracted to me. I completely understand that I am physically unattractive, and I will therefore seriously struggle to find a girlfriend.
I don't really see why male virgins are the target of so much criticism. I'm a romantically lonely male virgin (and believe me, that has a very serious impact on my self-esteem and mental health). I'm not hurting anyone. As far as I'm concerned, society should probably stop mocking and insulting lonely single men so much. If people want to criticise others, why not criticise people who are angry, people who hurt others, or people who are violent? I don't really see the fascination behind ganging up against lonely, pathetic, male virgins, whose self-esteem is already at rock bottom.