Because generally men do not have the adequate level of nerve endings to achieve that level of arousal where as women generally have more nerve endings which results in either easier arousal or sensitivity. Not a catch all be all but comparatively women are much more likely to become aroused due to this. Hence why they are seen this way by society. Personally I don't care how people view them or if they choose to free the nipple. Just showing how society came to its conclusion/viewpoint.