The self-checkout technology will never improve as long as that assumption remains. If you can't trust the customer to not steal a pack of gum, you can't trust the customer to not steal an entire beef tenderloin.
The professional cashier does not need to keep every damned item on a giant scale after scanning it, because they are an agent of the store, and presumably trustworthy enough to notice and stop shoplifters and scammers. And they usually don't have to halt the entire checkout process and wait for a manager to come by and restart it whenever anything even mildly out of the ordinary happens, such as using a coupon.
You can't improve the technology when the premises are wrong. And the premise is that you can turn a grocery store into a glorified vending machine.
Remember, the store is more interested in protecting its operating margins than in providing more convenience for the customer. If the technology does not allow a store exec to believe that it can stop a $30 steak from walking out the door unpaid, it will never be installed.