Which translates into the customer paying less

That's never how this works. Business profits go up long before customers see any kind of discount.

You get discounts when there's a surplus of something, and the purpose of these self-check-out kiosks isn't to create a surplus of grocery stock but to conserve and transform labor costs.

A lot of customers do love it.

I've never seen a customer's eyes light up with joy upon seeing a self-checkout aisle. Customers enjoy shorter lines, but when everything is self-check out you're still all in the same line. And self-check out is slower, so you're often stuck in a longer line.

The one upside of self-checkout is that you don't need staff to operate the machine, so if you're doing a quick in-and-out handful of items purchase during a slow day, it can be faster than waiting in the one line that has everyone in the store backed up behind it. But, again, that's because stores want to conserve on labor. Not because there's some fundamental benefit to self-swipe. If all the aisles were fully staffed all the time, there would be no discernible benefit to self-swipe.