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Large High DPI monitors are much, much harder to make because of the size of the panels. You get a lot more dead pixels for a panel that size.
Are you sure that the panel yield/dead pixels are the real limitation? Billions upon billions have been invested in making LCD production as economic as possible (manufacturers have built 10th or 11th generation production plants), but I think there simply hasn't been much demand for high DPI screens -- 1080p has become the standard not that long ago. When higher resolution content (4K video etc.) will become more mainstream, I'm guessing that the LCD industry won't have problems to catch up to make panels with the same resolution.