I don't know how things go in the US, but these days in my neck of the woods the vast majority of archaeological finds come from preventive archaeology. Pretty much every news article starts with "while making a parking lot" or "during the construction of the new XY highway".
My friend's job for a while was to justify not doing any preventative archaeology for new projects, as it obviously costs a lot to have to have 3 or 4 archaeologists go on site and do an excavation for a month or two.
It is interesting to note that from these finds apparently came the first T-Rex fossils with intact tiny arms.