Actually, the fact that you can differentiate an human character body right away when looking at it is pretty useful (gameplay wise speaking)
It was very confusing when all humanoid monsters had a death corpse similar to player ones, it was such a mess and "Look" festival!
Fortunately, when can say right away which corpse is from a player or from a monster nowadays.
Well, at the current state, Tibia got an immense amount of outfits available. Making death corpses for every one would be quite expensive (like already said above). I would particularly like it, but I don't know if that would be a worth investment.