The point of 165mm cranks is not hitting rocks, it's not how tall you are....
Is that you can pedal with higher cadence as track cyclists do... they normally run 162.5mm.....
that was always my impression also.
road bikes don't have clearance issues, yet they come with cranks less than 170mm. torque is traded for horsepower and rpm's.
downhill racers don't really need the slow speed torque that you would on a steep XC climb...watch most DH pro's and you'll notice that they are spinning the cranks at a very fast cadence...horsepower!!!