Pacific Rim, the movie spacifically.

[Hammerskjold]

The Kaiju's creators-- Moved from planet to planet, terraforming them as they went. Everything in their lives was a biological construct, including their buildings and machinery. They had waited for a hundred million years for earth to be ready for them and they were out of time as the planet they were on was out of resources and was dying. The destruction of Gypsy Danger destroyed the bio-machines that opened and maintained the rift. Without the rift, they're stuck on their dying world, doomed to perish with it.

Then they should really take a lesson from humanity on how well we take care of our own planet, and how well we manage our own natural resources, reduce and nearly eliminate pollution.

I mean just looking at our own real life version of earth it's clear that we're....oh...wait never mind.