In addition to the other replies, the weather there is really, oppressively hot. Going out in the middle of nowhere to search for sites requires lots of resources like fuel, water, food as well as the equipment to excavate sites. It's not like many gas stations or restaurants are nearby. So it's actually quite dangerous if you get lost. They must be funded really well with professional people so it often comes down to just how much money one is willing to pour into hunting needles in a hay stack.