I doubt their drivers are any better than anyone else's. Drivers are carefully trained and there are strict laws around driving trucks (much stricter than cars). They may do a bit more training than required, but there are other trucking companies that do the same.
I'd venture they are one of the safest[0] fleets out there which is a direct reflection on the quality of their drivers. Hell, I'd happily say their yearly average is better than the monthly stats from the last company I drove for.
The biggest determining factor on a driver is their safety record and if you can manage to not hit anything (you're not aiming for) over a long enough time period it is quite possible to get the good jobs.
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.
