^ Becasue you can't load a set of diff gears for breakin like engines are.
Never bought a semi so can't comment on them. But, to your point, vehicle will likely be fine if you tow with it when new.
I've had countless new company trucks. Never paid attention to breakin and several of them have gotten hooked to a trailer well under 500 or 1000miles and driven cross country like a rental. Never any gear issues in those or ANY of the other newer trucks I've had since I started driving.
There was one defective truck that had rear diff problems but it was grossly defective.
Bottom line, differentials are one of the more bulletproof components on vehicles and issues are rare, regardless of use.