Where are the Berserkers?

Berserkers ie militarized von Neumann probes are the most efficient way to explore and conquer the universe, yet there is no fleet of Berserkers currently sterilizing our planet. In the 13 billions of years since the Big Bang, in the trillions of trillions of planet, has there been no species intelligent and expansionist enough to build a Berserker fleet? If intelligent life is relatively common like some people claim, surely one of them must have built a Berserker? Or are we alone?

Maybe it's too fucking hard to come up with technological or biological systems that can cross interstellar space.