Thing is, they do respond, it's a known fact that they respond, it's why trading routes become useless, there is a demand, it starts getting filled, and that demand drops over time as it is being met more and more until there is no more demand. Rares are an example of a supply that isn't in demand near the point of origin but at farther distances, the demand exists, to the point where you'll make a hell of a lot more than 4k profit. Funny, but it seems to me that that's pretty much exactly how many real world supply and demand situations operate. I lived in Bremerton, Washington years ago, across Puget Sound from Seattle, a major shipping port, and prices for almost everything were really low due to all that merchandise passing through the port of Seattle. My mother lived in the Florida Keys and she came up to see her newborn granddaughter, I made her some coffee and she threw a massive fit at my foolish spending habits. I had made her some Kona coffee, which was around $36 USD a pound where she lived, and that was why she was so upset. I paid $1 a pound for it, which mollified her quite a bit. She literally bought as many cans as would fit in her suitcases and shipped all her clothes back home when she left. There were always lots of cans of that Kona on the shelf at the local grocery store, while at my mom's in Florida, it was always on back order, they couldn't keep it in stock, even at that incredibly high price. THAT is supply and demand, if something is in big supply locally, there's little demand, you have to travel a distance from the supply to start seeing a demand, and the farther you travel, the higher that demand MIGHT become, that varies depending on the product. I'm pretty sure there's little demand for Kona coffee in Columbia for example, or Nepal, but in the Florida Keys or New York City, very huge demand for a very limited supply.
I'm sorry you don't actually understand how these things actually work and instead rely on your extensive game acquired knowledge which is based on absolutely nothing in the real world.