If you assume that berserkers are difficult to produce and disperse throughout the galaxy, then you might conclude that they have limited capabilities. Perhaps

If you assume that berserkers are difficult to produce and disperse throughout the galaxy, then you might conclude that they have limited capabilities. Perhaps after detecting intelligent life and reporting it, perhaps it attached itself to the largest convenient asteroid/comet, and altered it's orbit just so to stunt human development just enough to allow for a more complete extermination ship to be dispatched. Like a galactic immune system - suicide berserkers designed to signal the human infection, and then suicide themselves to slow the disease evolution. The galaxy is many billions of years old. I would not rule out the development of such a system.