Au contraire! In Pacific Rim, they DID explain how the kaigu were on earth, and they also explained how and why the Jaegers were built. They gave both the kaiju and the Jaegers realistic limitations, and they even gave backstory to round it out. In all previous kaiju movies, the monsters simply "appeared", and shortly after that they made a beeline to Tokyo which was destroyed in almost every kaiju movie. Also, when the monsters died, they frequently either sank to the bottom of the ocean (to be revived later in another movie), or they were buried under rock and dirt, and were revived later. On occasion, they flew into space or morphed into something else, like Mothra.
Like I said, this is a genre unto itself. The rules of the genre were always very loose and these movies should be considered more fantasy than science fiction. But Pacific Rim changed all of that. Its the closest they have come to science fiction.