What is the drag of helicopter rotor at jet speed?
There were actually some experiments with stopped rotor. Furthest with the actual implementation was probably the Boeing X50 Dragonfly:


  By Source, Fair use of copyrighted material in the context of Boeing X-50 Dragonfly


The main reasons I can think of why nothing like this reached production yet are:


When the rotor spins, the blade on one side moves aft and needs to have its leading edge face aft, but when the rotor is stopped, both sides need the leading edge facing forward. So either the airfoil must be symmetric, but that has really poor efficiency, or some blades have to turn somehow, which is complicated.
To be light, rotor blades are normally very flexible and rely on the centrifugal force to keep them straight under load. But when they stop, they won't have such force, so they must be much stiffer, which makes them bit too heavy to spin.
Having separate wings and rotor would add a lot of weight, especially since the rotor would still have to be made strong enough to resist the aerodynamic loads when stopped.