Cause there aren't any in live action...
Sure, there are literally HUNDREDS of episodes of ultraman type shows battling alien hordes, animated films, Voltron, etc, but the genre in live action doesn't exist, Pacific Rim is a first. It's it's own genre.
There's a CGI film with robots vs monsters though I forget the name. There's Robot Jox, but no monsters, and it was cheap. Jet Jaguar in Godzilla sort of qualifies, but Godzilla is the star of the movie. Same with Mechagodzilla and Mecha-Kong. Infra-Man is a cyborg.