Here we go again...
By apple's definition, it has to have over 300 pixels per inch to be called retina display. Apple will have to double the resolution of current iPad.
Ebooks are just designed to read. iPad offers much more. They are not even in the same level.
Because of cost reasons? Yes. It's obvious.
Not to mention horrible battery life. If you run any OS or games at higher resolutions, you need GPU and CPU chips to back it up, period. You really think current mobile GPU/CPU out there can handle retina display on iPad and offer 10 hours of battery life? NO!
You obviously are just keep dreaming of fantasy stuff. Keep dreaming.