Why does Walmart hire out 3rd party companies?

Speaking for the Olney IL grocery DC, the WM trucks are not maintenanced at all DC's, just like not all DC's have WM drivers. Much more likely to find in house tractor shops at the GM DC vs the grocery DC.

In our case, our outside carrier did the trailers & our own trucks at the shop but were not allowed to work on the WM trucks. They went to either the Hopkinsville KY, spring valley IL, or green castle IN GM DC's.