Its called supply and demand, the GT3 is in high demand for the production is has being a GTx model. Frankly I don't think you are really interested in a GT3 to be proactive enough to get one, otherwise you would of had several by now in the span of those 20 years.

Also I don't see the problem with dealerships selling high demand models to their best customers first, that is completely fair, and good for those customers and those are likely not jackasses that don't know how to drive btw, what I don't agree is with some shady practices like dealerships selling to flippers (the jackasses) for a cut on the resale, that should be heavily punished by Porsche, though we know it isn't.