Because heavier vehicles cause more damage to roads, bridges, etc: * https://en.wikipedia.org/wiki/Fourth_power_law https://en.wikipedia.org/wiki/Fourth_power_

Because heavier vehicles cause more damage to roads, bridges, etc: * https://en.wikipedia.org/wiki/Fourth_power_law https://en.wikipedia.org/wiki/Fourth_power_law And repairing/maintaining those things more often does require natural resources and creates emissions (and waste). It also takes money, which could be used towards other things like education, healthcare, or simply lower taxes if we didn't have to fix things as much.