Because that wouldn't be any more useful. It would have to be "flies straight at X MPH air speed with Y degrees nose down at Z degrees hyzer with A amount off-axis torque in B direction at C RPM." Considering most have no way to accurately measure any of those variables when we throw I don't see how quantifying any of them would be useful.
It actually a lot less complicated that that. A disc that flies straight (i.e. not fading or turning) at a given speed is going to be stable at that speed regardless of angle or RPM, as long as it has had a few seconds after release to dampen any wobble and high/low nose angle. The only thing that would cause this to change would be altitude differences.
As far as coming up with fade or turn rates at low or high speed, they could just use a baseline RPM based off the average thrower.