Those super singles you are talking about only come in 22.5 rims. They save fuel, which is why truckers use them. I have heard complaints about hydroplaning in the rain. The cost is actually less than buying 2 tires on the big rig. After the initial cost of buying 4 new rims to hold the tires, there is a cost savings.
I would imagine one of the reasons no air ride seats is the air compressor. A big rig uses air for a multitude of reasons(brakes, works the splitter on the transmission, supply the trailer with air, etc.) If the oem's would put a built in compressor like a big rig, it would make sense to air ride the seat.