One word explains why. Greed.

I generally agree that self-checkout lanes are infuriating. However, there is one anomaly at a Kroger grocery store in Ypsilanti Township, Michigan (Detroit Metro). There is a cashier that oversees 4 lanes, and 8 individual self-checkout stations and that woman was born for the job. She knows how to corral people to each of the stations, responds promptly to issues that arise, and is (here comes...) friendly. I actually breath a sigh of relief when I approach the self-checkouts and see her standing guard. It's made me think that there is (or can be) a science to effectively managing these self-checkout lanes so that customers don't pull out their hair. Now, whether corporations take the time to address the issue is another question altogether. That being said, you can't control the people who use the lanes. More often than not, I always end up in a self-checkout lane with a person who appears to be discovering touch screens for the first time or who is confused by all those pretty icons on the touch screen. I've seen someone with no more than 15 items take as long as someone checking out with a cart full of groceries at a regular cashier station.