The Guardians of the Universe, the bosses of the Green Lantern Corps, have divided the universe into 3600 sectors, each under the jurisdiction of one (originally) or two (currently) Green Lanterns.
This is obviously absurd. But lets pause for a moment to consider just how absurd it really is.
Current estimates place the number of galaxies in the universe at somewhere between 100 billion and 500 billion. So lets say 300 billion. That means that each sector has, on average, about 80 million galaxies in it.
Suppose youre a Green Lantern scanning your sector for signs of trouble. Suppose further, absurdly, that scanning an entire galaxy takes only one second. At that rate, it will still take you two and a half years to scan your entire sector.
Just watch this video and think about trying to police the entire universe with a force of 3600 (or 7200) cops:
Now back in the 1970s there was some confusion at DC about the scope of the Guardians authority; while usually described as the Guardians of the Universe, they were occasionally described instead as merely the Guardians of the Galaxy (not to be confused with Marvels super-team of the same name). There was even a silly plot point once where the bad guys removed Earth from the Green Lantern Corpss jurisdiction by yanking it out of the Milky Way.
This certainly makes more sense, but still not much. The number of stars in the Milky Way is thought to be roughly the same as the number of galaxies in the universe; so itd still take two and a half years to scan a single sector, if scanning at the rate of one star system per second.
No wonder theres never a Green Lantern around when you need one.