A great deal of highway spending is subsidizing truck shipping. Most of the damage done to highways comes from big trucks, and taxes on them don't come anywhere near paying for it.

[EDIT] Incidentally, maybe we shouldn't subsidize truck shipping! But using usage taxes to pay for highways while not shifting the burden such that truck shipping is paying the lion's share of the cost and is no longer (so very much, at least) subsidized, would be making non-commercial drivers subsidize truck shipping (even more than they already are) which subsidy (to some degree) benefits everyone, including non-drivers, which seems like a weird and/or bad move to me.

Truck shipping has never made sense to me. It's significantly less fuel efficient than rail shipping, and paying people to just sit there and drive for their entire lives feels kinda torturous to me...

Lots of communities aren't served by rail, ship, or barge (or nowhere near enough capacity of those, or efficiently connected to the wrong places—how much raw iron ore or coal does a small town need?) so truck shipping's all they've got. Rail may pass through nearby, but without unloading capability or sufficient capacity at a proper train yard. Trucks drive to them from the nearest hub or notable train yard, which may be way more than your usual "last mile" concern that a city or whatever has. Tens of miles, to the warehouses that serve them, then tens of miles more to the nearest major store, that kind of thing.

Rural living would be way more expensive and inconvenient without truck shipping subsidies. Which, maybe making rural living artificially cheap is a good idea and maybe it's not, but that's one big shift that would happen if we made truck shipping pay its way entirely.

[EDIT] For true long-haul, I'm with you, the majority of that's surely only viable because of those subsidies, and we'd be better off pushing that to rail, ship, and barge.