Because the guy who they are going out with was once 'the hot guy' to them.
Let me count the awful roll call of ways in which you can become the hot guy:
Persistent exercise giving you a tauter build than most males, preferably accompanied with a side serving of unquenchable arrogance that might come from having a differently shaped body.
An expensive car / nice house / well paid job or , if it is only an averagely paid job still one that gains points with females for the perception that it gives of both risk taking AND security (what a contradiction) e.g. firemen, policemen.
An accomplished feigning of being in 'the friend zone par excellence'. Those males who slyly make themselves the ULTIMATE friend
and, in doing, learn both the girl's and their boyfriend's weak spots so that they can offer sympathy and immediate pick me ups in times of trouble with none of the wider picture that there might be. Then, after they have inevitably failed to resolve her boyfriend's overtly magnified, perceived, flaws (and secretly they didn't want to resolve them), the so called 'friend' become the new boyfriend.
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So after a while the girl has been indocrinated in to her boyfriend's ways and means. If she ever has any feelings for someone a bit less shallow in the way that they try to attract girls then it is only in a friendly, even pitying, way. Unless something very seriously wrong happens to her she will forever be attracted to 'the hot guy' in the room, however much she pretends not to be or however shabby a person the hot guy's real self may be. And if she knows the hot guy to openly or secretly be a git to everyone who is more truly sincere then frankly she deserves whatever he decides to give her which will probably partly involve periods of anger and unhappiness.