The entire 2.4GHz hand is around 80MHz wide and I'm not aware of any current chipset that would support that degree of spreading.
The thing is that modern frequency-agile 2.4GHz RC gear is about as bullet-proof as you can get (without spending a *lot* more money).
It's kind of like saying "why don't Bugatti increase the top speed of the Veyron from 431.07 km/h to 500km/h?"
Simple -- 431km/h is probably fast enough for 99.999% of customers and who would want to pay another $250K for the extra 70km/h or so?
Remember -- perfection is the enemy of "good enough"