With prosthetic makeup it’s a lot easier to give an actress a big nose or a fat face than to take away what’s already there. For simulating the effects of an amputation this is true in spades – a hook where a hand would have been is always going to be a hook beyond where the hand still remains.
Somehow the continuing existence of that inconvenient limb has to be disguised, so movie-makers often resort to a combination of techniques to create the illusion: an arm bound behind the back or a leg hidden in the wheel-chair; a prosthetic stump; a prosthetic hand that fits like a glove (eg Home of the Brave); precisely planned camera angles; trickery with cuts; CGI.