Everything about them is built heavy. I'm no expert, but i remember my mechanic friend saying that the cost of simple wear items was astronomical because of how beefy everything is. Brakes have to be big to stop a heavy vehicle.
Still, there are normal civilian vehicles that are big and heavy and still have to stop. Same for civilian vehicles having to lug a lot of weight around, bounce over rough terrain, etc.
Pretty sure they could have contracted the base vehicle out to Freightliner, Mack, etc. and gotten something that any truck stop could fix...once they figured out how to get to the standard equipment underneath.