Why is Apple Watch Workout calibration limited to outdoors?

If I understand what Apple is doing, they are having you walk or run outdoors (at different speeds for the most accuracy...several calibrations) to calibrate your watch for your stride etc. I live in a very hilly area so there is no way I can walk or run around here without going up and down a lot of hills. However, I can get on my treadmill like I did today and note that my watch was 18% low on the distance I traveled. Hence, why don't you let me input that distance for the calibration????