This is a very good point- consider all of the friction points that make self-checkout slow and cumbersome. How many of them apply to manned checkouts?

The weight thing is absolutely the most frustrating, and I would put money that it's not an effective theft deterrent.

I don't know if it's intentional, but the places around me seem to have largely solved the problem of cashiers being faster, by putting the slowest people on earth as cashiers...