They're placed on ground that's usually not being used or isn't being utilized in a better way. Ground solar is much cheaper than any other method and can be put together by almost any idiot.

I like the idea of building them over parking lots, too, though. Parking lots are big contributors to the urban heat island effect as those large black slabs absorb a lot of light and heat. So it would provide a bit of shade and help cool down the general area.

There is the risk of someone hitting a pole but you could just place them like we do with the street lamps - use a concrete base that goes up a few feet from the ground.