Turbofan engines have an inlet. This is a huge advantage, since with a properly performing inlet, the airspeed conditions at the fan face are pretty much independent of free stream airspeed.
Ignoring things like inlet separation and boundary layer effects, downstream of the inlet "throat" the inlet conditions are basically governed by the corrected airflow - which in turn is largely governed by the corrected N1.
Long story short, turbofan engines can get away with a fixed fan blade angle because at a given corrected N1, the fan will see a pretty much constant angle of attack independent of free stream airspeed thanks to a good inlet.