I would say that it is because of the high probability you already have a boyfriend. After all, if I'm interested, then everyone is interested and you must have already found your person. So 95% chance you are already taken, so why even try? This is bolstered by being unapproachable by making no eye contact and generally sending out the "do not approach me" vibes. What guys are desperately looking for are "choosing" signals. The first move isn't a girl going up to a random guy. The first move is the guy looking at the girl and seeing if the girl looks back. THAT is the first move. The first move isn't the guy going up to some random girl completely cold and asking for her number. It would be as simple as looking back and smiling to give us the "signal" that we can approach.

The bottom line is the reason men are scared to make the first move is because women so obviously don't want to be approached or at least that is what we presume by default. So many articles detail horror stories about women not wanting to be approached. So, you are getting exactly what you wished for. Be careful what you wish for.