Green Lantern has a magic space ring that can do anything, and it's his job to patrol an inconceivably giant section of the universe. So imagine you were a man, with a man's needs, all by yourself in space, with a ring that can do anything. Your imagination just conjured Man Comics Presents: Green Lantern: Star Pervert. Great job.
Sometimes it's just a matter of making the US Department of Defense look, like, REALLY cool.
Actual impending doom like global climate change or mass extinction just makes people bored.
In some cases, the Marvel source material just did better.