Why don’t more stores have a single line that feeds all the registers? Banks do it, airlines do it, and Barnes & Noble does it, but other stores seem torn. As this nicely-done WashPost piece points out, even when such lines form ad hoc in stores, as in the RiteAid example she cites, it is an unstable state: everyone scrambles for individual registers if someone cuts the queue.
Trouble is, while the response is rational, the line-crashing behavior is not. It is almost certainly faster staying in a single line and being able to go to the first open register — and stores know it. But they also know that customers get spooked when they see a long line — even if that single line is just the sum of many smaller lines spread over multiple regisers.
Stores have learned that people simply like to feel in control. They want to be able to choose among lines, even if it is not a sensible thing to do. It is, of course, the same illusory control that has many otherwise sensible people driving rather than flying, despite the higher risk of the former activity. We are wildly subjective about risk, the same way we continually find evidence for the Law of the Slowest Line (whatever line we are in is always the slowest).