Same, self checkout is just 9 times out of 10 the most convenient option for people who don't buy a cart full of stuff.
It's not like stores are hiring people just to man the registers either.. They hire X amount of people and if they need to open another register that's someone who has to stop doing something else in the store, so self-checkout just lets them always have a bunch of registers open with only one employee overseeing them all and helping people out.
At least this is how it works in sweden, maybe in the US stores do it differently since they seem to have great difficulty making something as basic as scanning a barcode work.