Cats are polite. When they go to the bathroom they dig a hole and cover it up. Cats are very clean. Dogs love stinky things. Cats are very dignified and know who to have sex with.
Cats stay beautiful all the time while dogs are ugly and get uglier.
My immediate reaction was that they were advanced mechas, and that the CE3K imagery was just Spielberg playing with our expectations of what aliens look like. There are plenty of clues in the film that they're mechas, and almost nothing that I saw suggests aliens, but I'm surprised by how many people thought they were aliens. That's 24 percent interest compounded annually.