Because that's physically impractical (if not impossible) with today's technology. Intel themselves actually have a good article[0] on the topic. Besides, there's way more to CPU performance than clock speed—today's Intel CPUs are several times faster than the Penitum 4, for instance, even at lower frequencies.
> @nVidia: how about thinking about affordable high end GPUs? The GPUs got 400% more expensive since 2011.
What are you talking about? GPU performance is plateauing a little bit, but the cards are only getting cheaper and more energy efficient, especially the high-end ones. Both Nvidia and AMD have announced new high-end cards coming soon that will give you the performance of the current high-end cards (Titan X and Fury X) at a little over half the price. Edit: Apparently they won't be that much cheaper than previous generations, but certainly within the grasp of any enthusiast or professional who really wants/needs them.
Plus, isn't "affordable high end" kind of an oxymoron? "High end" is almost by definition not affordable for most people.
[0]: https://software.intel.com/en-us/blogs/2014/02/19/why-has-cp...