To be fair, QTing on a mass scale is a difficult and expensive process. Diseases don't always respond to the "standard treatments" and when one fish needs alternative treatment, all of his tank mates are along for the ride. If I'm lucky, I can successfully QT a batch of fish in just two weeks. But if complications arise it can sometimes take months before the fish are all clear. Every situation is different, and realistically not all fish are going to make it when complications arise. Most LFS cannot absorb those kind of losses and stay in business.
So why should I ? Why should I spend money on sick fish ? Why should I give you money for fish that will die if not treated.