As long as people keep going to stores on these days they will be open. Why turn away good business?
Actually, because it's unproductive.
The fallacy here is that if stores are open it will add to business, and that's not always true. What it does is dilute sales by spreading the volume out over a great time frame and is actually less productive and more expensive. Retail sales volume, like many sales volumes, is not a function of hours of operation in most cases; you usually don't get 10% sales increase by being open another hour. Taco Bell staying open till 2AM to feed the drunks may be an exception, but lots of that is just a shift of dollars away from pizza joints and 7-11s; the market is generally the same.
If stores closed on Sunday like they used to be, your total retail dollars would probably remain the same, but sales/hour would increase, as would throughput and productivity. Hours would be lost for employees as there would be no work on Sunday, but that would be partially made up by the increased business needs the other six days. Unfortunately, some jobs would be lost as the efficiencies of scale would cause increased production.
But, retail is loaded with lemmings and if one does something "innovative", well the others all soon follow suit...