they do it because people have been talking about this phenomenon for decades now (well at least since the 80s).
so they get the idea from guys who always point it out. it's a vicious cycle. the best is when youre out with a group of peeps and you get up to go to the washroom, and one of the guys is all drunk and slurring "hehshshohhhheeeeeheehehhhrrr, arent you all gonna go togather?????harrrrarhaarrrrrrrrr. pfffft. women"