Speaking as a man who has never had a girlfriend (and, by extension, is still a virgin) I have to put a caveat here as all that I am about to say is pure assumption, and simply my own observations over the last 2 decades of my life.
Humans, despite our sentience and higher intelligence, are still very much primal beings. In our primal minds, we wish to mate, and we wish to mate with as many members of the opposite (or same) sex as we please, as is nature for several species. However, unlike other species, thanks to our own sentience, we cannot simply engage in sex with whomever we please. Rather, we need to engage in what we call "romance" in order to woo/persuade the person/people with whom we wish to have sex with that this is a mutually beneficial concept. This procedure can take anywhere from a single evening to several years, so in essence, we put more effort into the pursuit of sex than most creatures put into the act itself.
In this way, the "Friend Zone" to a lot of males is a deterring factor to our own primal instincts. I know it may not be true of every man in the world, but there are a significant number of men who will only interact with a woman for the express purpose of wanting to have sex with them. They don't care about who the woman is, their needs/wants/aspirations, they only want to satisfy their primal desires, so, in being placed in the "Friend Zone" these men simply shut down, in a manner of speaking. If one of these men realizes they are not now or never will receive sex from this woman, then they will no longer interact with this woman.
Now I'll say it again, I'm not assuming that every man is like this, nor am I assuming that the only reason men interact with women for is sex. However, there can be no denying that there are plenty of men like that out there, and there are probably some who will "bide their time" in the Friend Zone, trying to snatch an opportunity to rise up into the "Friends with Benefits" Zone. Also, there are probably plenty of men who put women in the Friend Zone as well. I, for one, have a couple of female friends that we're both in each other's Friend Zones because A. they live several states away, and B. they're either married or in committed relationships. I don't see this as bad, because C. while I think they're great friends, I just don't see them as potential sexual partners.
I could probably go on a lot more about how our own concepts of "Love" and "Marriage" hinder our own primal instincts, but I think I've made a clear enough point how much of an opinionated, overly-negative bastard I am, so I'll just leave it at that.