That's sometimes part of the awkwardness. My additional gripes are completely pedantic, but hear me out:
None of the grocery chains in my area offer bagging services, so I have to wait for the attendant to scan my items, bag them myself, and then shuffle back to the register to pay. None of this is a problem, except that the human attendant rarely ever scans the items in the order that makes sense to bag them. So there's the weird dance of waiting, pushing items aside, etc.
If I'm doing self-check, I can scan and bag using my own patented autistic process ™ that is quick, makes sense, and doesn't crush things, and I can do it in one shot.