As so very often in D&D, certain potential game elements are absent for a reason. Limb loss is one of them. Just like Critical Fumbles.
Ah, but critical fumbles aren't entirely absent. They're presented as an alternate rule the DMG, IIRC.
Originally Posted by The Doctor
People assume that time is a strict progression of cause-to-effect, but actually from a non-linear, non-subjective viewpoint - it's more like a big ball of wibbly wobbly... time-y wimey... stuff.