Same reason I usually don't go to self-checkout.

Yeah that's basically the same issue. I do think the self scanning is even worse than the self checkout because at least at the self checkout you scan everything once at the end when you're done.

The fact that you have to remember to scan every time you put anything in your basket is just worse for the mental load I find.

If manned checkout is a dishwasher and self-checkout is handwashing dishes in the sink after the meal; scan-as-you-shop is eating next to a little basin where you wash each utensil as soon as you're done using it, before you're allowed to return to the meal.

The 'little basin' is apt because the quality/usefulness of equipment is a huge part of why it always sucks. I used a scan/pay/go system in what seemed to be a trial rollout at a major supermarket. They used your choice of a handheld scanner that lagged tremendously compared to a real scanner gun, or a smartphone app (which of course is laggy AND clunky, since you have to keep unlocking your phone and tapping some button, and waiting for it to register)... I pretty much hated it. I only used it while COVID was a big deal. They ended up pulling it and not rolling it out.

A few seconds between dishes sounds pretty comfortable to me.

Don't forget to mention that with the manned checkout dishwasher, it's fast and powerful but you're stuck standing there until it's done.