You'll probably find that your laptop display has a comparatively low refresh rate (probably 60Hz) and much slower response times (8-24ms) compared to desktop displays which are often considerably faster. High-res, high density displays are also more expensive - quite a chunk of the money you spend on a laptop pays for the display.

Slow response times are unattractive to gamers, as they can cause ghosting, and lower refresh rates tend to strain the eyes. Given that, and the relatively high cost of producing a display of the kind you describe, I guess that most manufacturers simply don't feel there is enough money to be made from them.

why don't you get a 24" display and a bigger desk?