I should actually correct myself slightly: Wal-Mart (and others) have some in house drivers and some outsourced.
BTW, in discussions of the transport industry, don't get distracted/lied to by the companies. Some drivers think they are owner operators, when in practice, they aren't. They will lease/buy a truck from (as an example, all of the bigs do this) Schneider. As part of the lease terms, they can only accept loads from Schneider. It should be obvious that the 'owner' is an employee who has assumed much of the risk that the company would usually take on.
ShanghaiBill has a decent reply, but he misses a point: if the automated truck is cheaper, the big companies will drive that change in a heartbeat. The trick is that someone has to be convinced that they will be cheaper. They are unlikely to automatically accept that an automated truck is safer, faster, etc. One area where they are likely to be impressed is the possibility of 24 hour operations, rather than the 10 hour per day (rough) limits of human operated trucks. In addition to (possibly) being cheaper, this will allow faster shipments for more mundane goods (there are already plenty of ways to have fast shipping, but it is cost prohibitive to do for everything) which would offer them a competitive advantage. I suspect this last point will be the thin edge of the wedge.