Why don't many women/girls approach men/boys?

Maybe it is cultural difference, I don’t know. I am from West Africa. I have been in the America for a while now. One thing I have noticed recently is that many women/girls won’t or don’t approach men/boys. It is as if they want the guy to make the first move and approach them. I mean if you are interested in someone, why not just approach them. After all you never know if you will ever see them again. Where I am from women/girls are not afraid to walk up to a man/boy they like.