Because they don't want non-human characters to be visually or, god forbid, sexually appealing to audiences. The absolute LAST thing a studio wants to hear is that that werewolf was was kinda sexy, or that alien bunny was actually sorta hot. If it's not human or is some sort of crossbreed, it has to be portrayed as a monster of some kind. It all comes back to the morals brigade, and we know where the morals brigade is rooted.