Because with AIM, it's not real communication. If you talk purely with AIM, or mostly with AIM, the girl will assume you don't have the balls to chat in person and will immediately dismiss you.
One girl I talked to over AIM for a year put me in the friend zone, and we are good friends now, but it's taking very long. Another girl, very similar I talked to in person for less than a month I asked out, and she is currently my girlfriend.
There is a big difference. To this day, I have been on this site for exactly two years. I have been in MANY AIM conversations. I recently made a very popular friend, and in the few months we've been friends, I've learned more in one day of hanging with him than I have in a few months of reading here.
NOTHING is worth more than experience. AIM isn't experience, it's reading. Go out there and make it happen. First, do it with chicks that go to other schools. Make one good female friend outside of your school, so if you mess up and do stupid stuff, no one at your school will find out. And then, she can evaluate you and you become better.