With such a variety of modern metal detectors, what are the chances that the dealer will have the few detectors (or even any), of the detectors that someone wants to try out? Even if they did, does that mean that person can hunt with the detectors to test them out? What does the dealer do with the detectors that were tested and not wanted? Sell them as demo units and take a significant financial hit? That is of course, unless the manufacturer provides demo units free of charge.
Above and beyond that, a dealer is only one person, and that one person may have bias. They also may not have full knowledge of all the various detectors and hunting ground types. As such, it seems to me that by far, the best way for a newbie to learn and decide upon a new detector, is by using the detecting forums to get good information from many experienced and knowledgeable hunters.