A lot if the time people who bag groceries that aren’t cashiers also clean messes, gather shopping carts, face aisles etc, so if it’s a huge supermarket they’ll probably be happy to skip that job.

Actually when i was a courtesy clerk (which included bagging,) bagging was my favorite job. I got to stay indoors, lean on the checkout thingy, chat with the cashier, etc. I don't know if they still do it, but safeway would schedule you for 30 minute rotating shifts. Some would be bagging, others carts, others still would be "free time" where you walked around and did whatever people asked of you. I always hated when I'd come in and I'd only have two or three bagging shifts.

So if you've ever seen a bagger just walk away or randomly appear out of nowhere, that's because their shift either ended or it just began.