Exactly, so why NOT use Superman villains who aren't Lex Luthor, or Zod?
Probably because most of Superman's other villains are a lot more weird and fantastical than Zod.
Nolan and Snyder have talked a lot about making this a more "grounded" world, so having Supes go up against killer robots, giant aliens, or monstrous mutant versions of himself would probably defeat that a bit.
At least for the first movie. I'm sure we'll see some version of those other characters in the sequel.