Why are the last set of wheels of ten-wheeler trucks not touching the ground?

You're looking at a truck with an auxiliary axle.

Some trucks are permitted to weigh more than 80,000 pounds. I'm in Idaho so let's use those numbers. The maximum weight of a truck in this state is 105,500 lbs. To do it, you need at least seven axles on your truck and you're better off with nine - five axles on the tractor, four on the trailer.

If you don't haul 105,500-pound loads all the time, having those extra wheels on the ground when you don't need them down only wears out your tires, and tires are expensive. So, if you're a driver who pulls some freight that only requires five axles and some that requires all nine, you have your truck equipped with a hydraulic auxiliary axle. By moving a switch in the cab, the driver can lift those extra axles off the road when they're not needed and save the tires.