I don’t think dump trucks have air suspension. Buses, regular freight haulers, sure… but gravel and rock don’t care.
There’s also a big difference between air-adjustable springing and height adjustment, which tweak settings but don’t change the suspension action much, and having suspension that is both ultra-heavy-duty AND can be swung up out of the way.
It might be simpler than I’m envisioning - never looked closely at the machinery - but it still seems like a lot of troublesome engineering and maintenance and so forth for trivial reasons. Unless “more than three axles” are not permitted in places some dump loads have to go - or some such.