If everything was smooth and regular, trucks wouldn't have a problem. However, the roads, and the warehouse yards, are not.

A fully loaded truck traveling 55mph (on some roads they are allowed 65 or even 75) can weigh 80,000 lbs or more (in Canada under certain circumstances, 139,500 lbs).

That is a tremendous amount of force that must be handled properly 100% of the time, through rain, hail, sleet, snow - and with smaller cars weaving in and out of traffic all around them.