Lets say they will. How is conquering a far off enemy worth it economically? It would be easier from a resource, political, technical, and moral perspective to build out large solar arrays or build more nuclear plants (or more fantastically, build out a Dyson sphere) than a space war. We're not post-scarcity, nor even close to it, but we certainly have better options than mindless destruction for energy. Now imagine us 5,000 or 10,000 or 100,000 years from now. Why would we fly to some far off planet to murder everyone for their resources? The universe is teeming with energy and resources. Its simpler and more ethical to just tap local-ish uninhabited resources.

Arguably, if your species is mind-mindbogglingly stupid enough to declare war constantly and against the basic rules of game-theory then I doubt your society will survive the nuclear weapons stage anyway and won't ever be in the stars. The nice thing about the Drake equation is that it suggests that Klingon-esque societies don't make it to the space-faring stage, which is nice.