I didn't say they aren't important, but I did say that they are not responsible for the highest profit margins.
And answering your question, because it's not cost effective. The highest end chip obviously costs more to produce then all the others that derived from it. The R&D costs also revolve around the highest end, since from the design of that one, more chips will be created. High end cards are not meant for high volumes of production, so they have a high price for longer than others, which usually is justified because they perform better. And that's true, but for money making companies what matters is how much it costs to produce one and how much profit they can make out of that one. Since they sell less of those, they charge more for them and that's the reasoning behind "Supply and Demand". They could probably price it well enough for more people to buy it (high end I mean), just not enough to keep a good profit margin on it.
Just look at what happened with the 7800 GTX 512 and 8800 GT when they first debuted. The 7800 GTX 512 was obviously just meant to spoil the already late arrival of the X1800 XT, but NVIDIA didn't anticipate that people would flog to them as they did, even at the ridiculously high prices it had. Same for the 8800 GT. Launched before ATI released the HD 3800 series, to spoil the launch once more. Only this time, the 8800 GT had a very attracting price, which again, led people to grab it off the shelves and click on the checkout button like maniacs. NVIDIA had to increase orders to TSMC because of that. There was a piece of news about that. I'm sure you can find it.
Well, I'll just leave it at I disagree with your reasoning.