It only takes 1 employee to staff 20 self checkouts for comparison.

For a full fat grocery store. With zero change or adjustment to the rest of the grocery store. And customers weirdly like self checkouts even when they are a dramatically worse outcome (compared to the highish bar of well trained cashiers)

We like self-checkout because there's hardly ever a line.

An idle self-checkout machine costs the store almost nothing. An idle cashier costs the store wages. So the stores will always skimp on cashiers, leading to lines, wasting my time.

What exactly is the point of a well trained cashier, what service do they provide. I guess I appreciate what the bagger does and the cashier knows the codes for the loose vegetables but those are minor benefits in my opinion.