So, we see in GoF that wizards — albeit a very powerful wizard — can conjure better hands out of thin air — or at least, more powerful, and put it onto a handless stump.
Voldemort raised his wand again and whirled it through the air [...] formed itself into a gleaming replica of a human hand, bright as moonlight, which soared downward and fixed itself upon Wormtail's bleeding wrist [...] now attached seamlessly to his arm, as though he were wearing a dazzling glove. He flexed the shining fingers, then, trembling, picked up a small twig on the ground and crushed it into powder.
So, through this text, we clearly see that wizards, admittedly very powerful wizards, but nevertheless wizards can conjure better hands out of the air.
Why don't all wizards just cut off their hands? Would it be too advanced magic for most?