I touched on it in the 3D thread.

As TV sizes get bigger, the pixel sizes get bigger.
It's somewhat easy to fit smaller pixels and up-convert HD signal into UHD.

After all, you can walk up to a TV at BestBuy or Costco and actually see the pixel size.

People buy 4K TVs because they don't cost that much more, marketing has done a good job on conveying the 'value proposition' and there is a perceived value in future proofing, even if there is no viewable content today. (sort of like 7.1 or 9.2 in AVRs)

PJs, as explained above work on different technology.
Getting chips to deliver 4K content is expensive and since there is not a lot of 4K content, the value of 4K in PJ world is purely marketing based.