Now I don't know about American cashiers but over here they're faster, waiting in line is generally faster, unless you're literally only buying three items. It's just better optimised overall: Waiting time is often cut to practically zero because you're arranging items on the belt while you're technically waiting, they're faster ringing up, and while they're doing that you can already pack so the second they're done you can pay and fuck off.

My neighbourhood supermarket has three self-checkouts, most of the time noone is using them because the one open register is always faster. In peak times they open a second one you might, in principle, find a window between that and the line of the first one becoming longer where self-checkout is faster, but I won't even consider it with a full backpack of shopping. Peak times directly before a weekend, or worse holidays are where the self-checkouts actually see use, as in you might even see someone waiting for one to be free.

Also you can't self-checkout best-before rebate stickers so there's that.