Any reason why we don’t put as many tires on (existing) trucks as possible? Seem like a good deal reducing tire wear.

It would probably increase wear, not decrease it. When a vehicle with more than one non-steering axle turns, some of the tires must slide. It's mostly not too noticeable unless the corner is fairly tight or the trailer has at least three axles. You don't really want any more wheels on the ground than you need to support the load. Or you need to allow those axles to pivot.

Heck, even with my modest two-axle travel trailer I leave tire marks on my driveway where I have to turn sharply to get it onto the road.

Increased friction -> uses more gas/energy. Complicated the mechanical design; more moving parts you have to maintain in a more crowded area.

It will complicate the control of the truck and add more components (increasing the cost).

Truck drivers don't have to pay for the negative externalities of tire wear