system
Given the following post, respond as if you're commenting on Reddit:
human
[40k] Why aren't Necron Tomb Worlds blown apart with Cyclonic torpedoes as soon as they're discovered?

It seems far less risky.
gpt
Usually the tombs aren't discovered until there is already something of value built on the world. You don't want to blow up a valuable forge world or agri world just to eliminate a Necron tomb. Planets are valuable, and the Imperium is not as trigger happy with Exterminatus weapons as some people claim, for good reason.

Also, unlike Orks or Tyranids that will sweep across a sector, Necrons tend to just go back to sleep when the finish eradicating life on a world, so the tomb world is generally not a big threat to anything other than itself. Blowing up a Necron tomb world to stop the necros would be like blowing up a building to save the hostages trapped inside.