A lot of women I've talked to simply feel it's not SAFE to approach guys, even if they want to; even asking out guys they already know. A lot of men, conversely, seem to expect that if a woman is asking him out that means she must want to have sex with him and get all offended when they don;t. This has led to my friends having experiences such as going up to talk to a guy, finding out he's not who she wants, and then being yelled at, called a bitch or a whore, harassed, followed...?
"I'm afraid for my safety because I don't know how a guy will react and whether or not he will turn violent" is a very compelling reason to not be approaching them.
Of course, I also know women who will approach whatever guy they want, they just don't give a ****. Like Nilla said, there are many possible reasons for women not to approach men and to be honest, I think it's unfair to complain about women not approaching men when there are many, many women who have ample reason to feel unsafe doing so. (Not that you're complaining, I know that you were just asking the question, but I do know men who complain endlessly about it)