Like most, I say it's not an instinctive male vs female behavior necessarily. I say it's due to two major factors: gender roles and demographics (the fact that girls outnumber us 7-1?? I don't know the number, but it's pretty bad)

It's established that what catches your attention first in an individual you don't know is their physical appearance. As Rêveur said, a lot of the prettier boys are jerks. This is because, as men, women also appreciate the good looks. So these guys have the added advantage to get the first step in with a girl and then they just have to not screw up too badly. Of course, initially relationships are not so profound as they are about making out and enjoying each other physically. Then they guy can say, "okay, well why keep this girl around when they're so easy to get and there are so many more worthy candidates"...after all, there are about 7 or 8 girls to every guy, which may be part of the problem. So the pretty boy is a jerk and girls learn that after a while. So then the girl starts to value other things more than aesthetics. By this time, us average looking guys have learned to make up where we lack physical beauty by being witty, attentive, and whatever other intangibles you can think of. So the average looking dudes get a shot at pretty girls thanks to the jerk-pretty boy's set up.

Now, as far as average girls not getting the attention...Since there are so many more girls than guys...there will be many more good looking girls than good looking guys..leaving a bigger pool of pretty girls not taken by guys for the average man to choose from. Why would he go for the average girl first when he can try his luck with a couple of good looking ladies? Because gender roles say that the guys does the choosing. The average girl doesn't do anything to get herself out there. Girls have not been taught to make the first move. Maybe average girls, too, have learned to make up the lack of hotness in other ways, but if you have to wait to get approached to show this to guys, you may not get the opportunity because in the mean time the guy is out there testing the hot-girl waters.

I'll stop there.

Freddie