That's a bit much to expect out of evolution. First it needs a way to detect whether a limb has fallen off or otherwise failed. Then it needs to send a biochemi

That's a bit much to expect out of evolution. First it needs a way to detect whether a limb has fallen off or otherwise failed. Then it needs to send a biochemical signal to the cells in the area iff the limb in the area failed. Then it needs to activate the regeneration iff the chemical signal is present. And each of these mutations has to be beneficial to the organism (or at least randomly there and costless). You can mush together the detection and signaling by having a local signal (like the biochemistry responsible for scabbing - iirc what happens is blood would do interesting chemistry while bleeding anyways, and so "turn into a scab when that chemistry is happening" is a worthwhile mutation), but still, it's a few too many steps.