Because it's not. You have it backwards.
The shorter the torso and the longer the femurs, the more bent over the torso has to be to keep the bar over the midfoot. People with short torsos and long femurs are not going to be upright back squatters.
In your mind's eye, picture a short torsoed, long femured person squatting. Now shorten his femurs and lengthen his trunk...do you see how he will be more upright as his proportions change with the bar remaining over midfoot?