Why the rubber?

We just returned from a 17-day 3,509 miles trip to southern California and points in between (on which, maybe more later), but after seeing the remains of at least one shredded tire every ten or so miles on Interstate or state highway, I need some information. The trucker is probably using retread tires, I understand that and why he or she might do so. My question: what happens to the truck when the tires lose their rubber? I kept looking in borrow pits and down mountain sides looking for the rest of the truck and most of the time I never saw one, even on the side of the road, no matter how fresh the detritus looked (even if it was all still in the center of the lane. Can anybody help?