I'm not sure that's really true. What would it take to wipe out all the denizens of a star system? Maybe a few million atomic bombs? Certainly less than the mass of a small moon. And if that then gave you control of a whole star system's worth of matter and energy, it should be more than enough to fuel the conquest of the next star, and so on.

So it's certainly feasible for an advanced race to build a star empire; the question is why they'd want to do it. If we're talking about humans you might say ambition or imagination. If Vulcans, maybe they'd think it logical to pre-emptively become stronger than any potential aggressors.