I don't think there are "lots", but I think there are enough to distort the view of the labor pool. Additionally, there is the issue of these fakers staying in the pool much longer and spraying their resumes out more often further distorting the view.

As for how they get through the screen, I think they just blatantly lie on their resume hoping to get a company that doesn't do its due diligence in determining if the stated experience is real.