Fascinating stuff. No scientific basis for this, but my obvservation is that running on a treadmill is much easier (even on an incline) than on the roads. Camber, incline, surface variations must all help to strengthen knees, ankles, hips etc
Treadmills were great for getting me into running again as a beginner - I could set the incline high and walk fast to achieve a workout at a time when even slowest running made me dizzy and ill. Now I can run outside and get somewhere I now only use treadmills to pace a short speed session or to warm up for weights.