How come so many girls I know who say they are Muslim wear low cut shirts and short skirts?

My best guess would be that they are merely following a trend, not a real religion. It must be popular in some places to say, "I'm a Muslim", while in fact they are not. If they went to Iran dressed like that they would no doubt be stoned. They are, in my opinion, making fools of themselves. But they are free to do that if they want to. Good luck.