I can remember a time in my young adult life when stores did not have this problem because store helped you out with your groceries.
Ah but there is more behind this. You go lets say to Wally World and buy a cart full of groceries. All cashier the two open that is are backed up forcing you to self checkout where 3-4 employees who should be running cash registers and bagging your groceries' are standing there watching you like a hawk until the machine screws up then they hide while you wait for them to clear it.
Now lets get to the real pet peeve which is who gets full service? Who does the store pander too and treat like royalty? I'm talking about people who shop at home, pull up into a RESERVED FOR ON LINE SHOPPERS space and their groceries not only are shopped for them but bagged and loaded into their vehicle for the exact same price I pay if I walk in and shop, then checkout and bag my groceries, load them into my vehicle and find the way to their cart return which is usually so packed it's out in the parking lane. All because all employees are too busy pushing the huge shop on line carts through the store blocking aisles, shelves, you name it to go out and bring in carts. Go back to full service walk in stores and the unstowed shopping cart issue will be resolved. A simple solution and no research needed just some common sense.
I prefer self-serve for groceries, especially anything perishable. I either am pretty picky about produce, or want to pick out the furthest out expiration date on an item I can.