What is it about retail, particularly the grocery industry, that makes it so hard (apparently) to executre at a store level?

I’ve actually read several studies that suggest that having more people (i.e., better customer service) yields higher returns for retailers.

Think of Trader Joe’s. Think of Whole Foods. Think of HEB.

Then, why, do a lot of retailers lack the appropiate customer service and are so inefficient at store level execution?

Are they trapped in a vicious cycle?  How, if at all possible, can they get out?