That face-to-face interaction is unnaturally reducing the performance bonus of management. Think any of the headcount savings $ goes to stockpeople?
A Home Depot person always walks by me in line, saying "Self-checkout is open!" to which I always reply "Never." Craig Menear doesn't need those micropennies for anything other than scorekeeping with his golf buddies.
But these are all just rationalizations. Self-checkout was a broken hassle years ago when I first tried it. I'm not under the impression much has changed, and my sympathies have grown with the labor argument. This seems especially evident in the lack of baggers (and bagging skills) at my local large grocery store. I think they stopped teaching it, but regardless The headcount-reduction engine doesn't result in a business I want to give more money to.