Why is the rotor system of an aircraft and the rotor system of a helicopter different?
The aircraft hydraulic pitch control system is much simpler and has fewer moving parts than a helicopter's main rotor pitch control system. 

A helicopter's main rotor runs at between 500 to 800RPM, but (for example) in the world of general aviation using piston engines an airplane's propeller runs at between 2400 and 2600RPM. If built to withstand the dynamic loads at 2500RPM, a helicopter main rotor would be far more massive.  

Airplanes have no need of the cyclic pitch control system that helicopters require in order to be controllable. 

All these things mean that mounting a collective-and-cyclic blade pitch mechanism on the crankshaft of an airplane engine would be a meaningless exercise.