I would rather spend (modestly) more time checking my own than less time standing idly with nothing to do but watch some kid checking out my goods. It feels better to be an active participant. Where it breaks down for me and my 12 items is when all the self-check lanes are clogged with people trying to ring up a full cart of groceries, who still haven't figured out how to work self-checks, who are encumbered by a baby in one arm and a phone in the other hand, or who just can't move all that well.
Managers using the presence of self-check as an excuse to understaff the actual checkouts makes all of those problems worse, and makes the checkout process suck for everyone.