Because those sites are businesses and they exist to make money. If you want to meet a girl, do it in person or...I did see a personals site recently that swears their services are free. The name is Singlesnet.
http://landing.singlesnet.com/geo_landing/hetm1/?OVADID=29766605511&OVKEY=free+personals&OVKWID=17563380511&OVMTC=standard&OVRAW=free+personals&adgroup=913423399&adid=29766605511&campaign=318936011&campaign_id=2484&display=Free+Personal&keyword=free+personal&redirected=true&searchterm=free+personals&type=std