> Giant brainless animals roam the ocean looking to sting 
  > and absorb prey without ever really knowing they're even
  > doing it. They're pretty much ocean zombies.

They do lack central nervous systems, but they're not entirely brainless. There's a non-encephalized diffuse neural ring that controls effectors which is sensitive to sensors input. Eerily, some jellyfish (box jellyfish, IIRC) have corneal eyes...

So, sure, these things are super stereotyped. But so are many higher organisms, even among good ol' vertebrates.