We can't easily pack humans more densely into trucks, so reducing their miles traveled has a much bigger effect (and if the distances are shorter they can also walk/bike/bus instead of single occupancy vehicle).
We basically can, with decent urban transit and rail services.
An articulated bus has a curb weight of about 35,000 lb and a capacity of up to 92 passengers. At 177 lb each, that's a gross weight of 51,284 lb. (net: 16,284 lb of passengers).
I'd presume a potato's density is close to that of water. As such, capacity is actually mass rather than volume-constrained --- you'd be unable to fill a truck to its volumetric carrying capacity before exceeding gross vehicle weight limits.
Many grocery products have much lower density: chips, bread, paper products, etc.
I still tend to agree it makes more sense to move goods than people, but the differential may not be quite as great as you might think.