In theory, you are right. In practice do you know many salespeople (under the age of fifry) who have knowledge of, or an interest in the items they are selling? Unless you find some old boy who has been in the business for forty years or more, nobody is able to answer you any technical questions and just stares at you with total incomprehension.
If we're talking about a big box or department store, I agree with you, it's often difficult or impossible to find knowledgeable help. However the good specialty stores that I frequent (one a fairly large chain that has several mall locations) have excellent salespeople that generally know what they're talking about. There was one time I had a very lively discussion with a late 30s salesperson about the merits of floating canvas vs. fused (with respect to weather and puckering) who knew quite a bit about pattern making, fitting, alterations, etc. Another time I hit upon someone else who could not answer my question (I don't remember exactly, but I think it was something about collar stays on their line of bespoke shirts) but at least called someone who had the correct information.