For the shop once stuff gets to the point where it makes sense to dump it like that, then it does not pay to have an employee sort it.
For the customers, they get a sense of accomplishment by sorting through the stuff and coming up with just the piece they need. People like tresure hunts.

It also raises the chance that the customer will find something else interesting and buy that too, whereas if it was all sorted and they didn't need to hunt, then they might just pick up what they want and ignore the rest.