[chmod600]

That leaves the question of why they pay 2x more, then?

Maybe fewer road incidents, reducing liability? Like, maybe people who can provide for and spend time with their family have 11.3% fewer crashes?

(Edit: number was made up, hypothesis purely speculative.)

[bluGill]

Because as others have mentioned, WalMart cannot afford logistic problems. A driver who knows they cannot find a better job will not bother looking for one. Thus that driver won't quit and have to be replaced on short notice. Hiring new drivers is difficult even if there isn't a shortage.