simple answer...alot of guys know that some girls get hit on constantly. And the chances of being successful with a pick based on a glance while on the street is rare. The possiblity of rejection in public can be hard on a guy ego. So the risk may out way the reward. You have to send him an invite if you want him to come talk to you. Just a return glance is not an invite...once an invite is sent then he knows his chances are better and the reward is now well worth the risk.
I think they either have girlfriends or I was pretty enough to look at but not pretty in their eyes to approach. I think guys have to be really really attracted to a girl to approach her, if they're only mildly attracted then they'll look from a distance but not make the effort to talk to her