I've noticed that the line for self-checkout is usually roughly the same length as the lines for individual cash registers, regardless of how many machines there are.

I'm not sure if people just don't like self-checkout or if they have failed to notice that the "go to the shortest line" heuristic doesn't work anymore when one of the lines moves much faster than the others.

In fact, just follow the logic down that road. Consider highly-volatile demand purchase scenarios. A company might over-spend on self-checkout stations to cover high-demand scenarios that go unused on more typical days. A more efficient approach is to avoid requiring additional stations to cover changes in demand. Just create a single system (walk out purchasing) that handles all scenarios and be done with it.

Although I agree just-walk-out scales well with demand.

There's almost always a line at them when I walk by in the stores in my area. Usually, the line there is about as long as the lines at the manned checkouts.