Primarily because they reek of it themselves; it's a way for compensating for their own lacks and faults.
That's really besides the point, but yeah, it's true. Women are very prone to what others think and that's just the way they are.
One of the main reasons why it's a challenge guys to get laid sometimes is because of the insecurity felt by women. Women are passive when it comes to initiating sex because they are SCARED shitless to labeled a "slut" or even to be associated with sex. Women put up barriers that filter out the needy, so only the most persistent and indifferent guys who're often just very confident, and at times could less about the girl actually get their pu ssy. This is because it's the confident men who think they've got nothing to lose are the ones that appeal to the woman's emotions. Nice guys who're more concerned with the girl actually come off as needy and therefor the woman automatically picks up this weakness and outs him because 1) he appears to have low value 2) she actually considers him manipulative because of his outward kindness which is accompanied by too much nervousness. Nice guys also try and be rational with chicks which absolutely kills attraction because when chicks think rationally they cannot be brought up to that emotional level needed for sex or even a decent convo.
You can honestly blame society for the conditioning and role given to women, or maybe you can just blame biology. I personally think it's both.
Most of this stuff is common sense, but most guys would rather blame money, social status and other stupid shit. Trust me, that stuff counts, but it's no excuse. If you don't smell and are decent looking and can execute talking to a chick with confidence and not caring about her reaction then you can get her number. If you can further talk to her and appeal to her emotions while being assertive in how you handle her and yourself, then you can fuck her easy.
Common sense, but difficult to execute and against conventional shit that we men were brought up upon. You don't need fancy dinners or flowers unless the bitch is already your wife or you are rewarding her for doing what you want.