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.

The fact that the galaxy is billions of years old is one part of what would lead me to think that such an immune system would destroy the entire planet, or at least kill all life on it, with the very first projectile/attack.

sounds like a scenario of Starship troopers rather than a food for thought.