If you don't trust me to the point where you're going to point 2 security cameras at me while I checkout, and this is your idea of "more efficient", then I can grocery shop somewhere else.
Cameras are there to lower insurance premiums. That's why for years and years every security camera video looked like it was filmed on a potato because there was no incentive to upgrade. The rise of phone cameras has caused the price of 4k cameras to plummet though so they are getting better.
Computer-vision has been a thing for years, though it's now bundled under the moniker "AI". I know that the self-checkout cameras at the store where I go use AI to "watch" customers. I would assume most of them do, now. Heck, the cheap camera that I bought to point out my front window for fun pretty reliably detects animals, vehicles, and people.
Sure, what you've described is cost efficiency for the store. I think the person you're replying to is more concerned with efficiency in their shopping experience, which won't be improved by lower costs given that the savings will just be funneled to corporate execs and board members (depending where you're shopping).
Would you rather have 2-3 lines with cashiers or 6+ self checkouts?
I would say I would get out of the store way faster doing self checkout. I call that efficient.
Places like Sam's Club with their Scan and Go checkout. I can scan all my groceries with my phone as I put them into my cart. Then I just walk under an arch with cameras that scans what barcodes it can. Then I walk out the door. I don't even have to wait in line or go to a self checkout.
Cashiers aren't efficient
If the company had to pay cashiers, then the prices of goods are just going to go up. Corporate execs and board members want the money.
So, do you let the Corporate execs and board members save money and hope not to raise prices (maybe lower them), or make them pay a cashier so they are going to raise prices.
Even though I hate the "please place the item in the bag!" "Unscaned item in bagging area!". Self checkouts are efficient, and the future.