Outside of Silicon Valley, not a lot of companies will pay you less under the auspice that the fringe benefits somehow "make up the difference" and Walmart is about as far from startup culture as you can get.
Aside from that in order to drive one of these trucks, you require a CDL, which you have to get on your own, and maintain on your own, all at your own expense. The pay is there to attract talent from a limited pool of available workers and to ensure they show up to work on time and make deliveries on time.
It would be against their own interests to try to cut corners and pay drivers less based upon these types of "benefits."
The point is that a delayed truck is very expensive, so it ends up being cheaper to pay efficiency wages and have good working conditions so people aren't constantly quitting and therefore delaying your trucks.
Wal-Mart was also a pioneer in just in time delivery for retail. If your supply chain is reliable you don't need to pile merchandise as high in your stores and your warehouses. People will drive an hour to Wal-Mart(s) in the most rural locations and in that case it is a real bummer to be out of stock.
I'd contrast that to K-Mart where I went to get 3 digits for my mailbox and could only get 2 of them so I didn't buy any.
Not if high turnover means slower deliveries which likely effect everything downstream (or upstream).
When you don't pay well, you probably miss out on the hires that learned all the skills, what corners to cut, what corners to not cut, what actual lead times are, etc when hiring.
