Yep, twenty registers with only two open...it drives me crazy!
After observing this phenomenon for some time in the few retail establishments I still frequent (mainly grocery stores) I can only assume the MBA types have this down to a science: Just how long can you make a customer wait at checkout before they'll simply walk out in disgust? Then manage your cashier staffing to hew to that limit as closely as possible anytime day or night to reduce staffing costs to the absolute minimum.
What's particularly galling is that now that they've introduced self-scan checkout systems, they include them in the equations as well. For some mysterious reason one or more are always down for "maintenance" or some other excuse so that customers don't get an unexpected taste of swift service to skew the expectation level which must be carefully managed to keep checkout frustration at the just-below-boiling-point level.
They should redesign those “unused” checkout areas for more product. Would be a better use of space.