> But while such a civilization would be energy rich, wouldn't planets in the habitable zone still be scarce?

Probably not. There's around a billion stars in the Milky Way galaxy alone, and even more in nearby Andromeda. If some civilization has the technology to travel to our system (which is likely tens of light-years away if not much farther), then it'll also have the tech to go to many, many, many other systems other than ours, which also have habitable-zone planets, and are probably more conveniently-located.

The only real exception is if there's a race that's not all that advanced, and is located very, very close to us, such as in the Alpha Centauri system, 4ly away, so that we're just close enough to make travel here feasible for them, but other habitable-zone planets are significantly farther so that it makes more sense for them to attack us rather than just find someplace else.

But as we're already figuring out now, a mere 4ly is already a really, really long distance with our technology. Our longest-range probes have only gone a small fraction of that distance, and they've just about left the system. It'll be tens of thousands of years for them to go that distance at their current speed. So traveling that distance is already rather unimaginable for us without something like a generation ship or cryo-storage ("suspended animation"), and when you have that, why bother fighting a war if you can just continue on?

Finally, if you have the technology to build a generation ship that can work reliably for 10,000+ years supporting life, you probably have the technology to mine asteroids, build habitats on the Moon and Mars, build large habitats with artificial gravity (by spinning) in Lagrangian points, etc. In short, you already have the technology to make habitats perfectly suited for your species, and there's a whole solar system full of building materials at your disposal. If you have that, why would you want to travel such a long distance just so you can steal a planet from some other race, when it's very unlikely that planet will even be suitable for your biology without significant terraforming?

In a nutshell, the problem with this kind of thinking, and a lot of sci-fi in general, is that it assumes that interstellar space travel is much cheaper and easier than artificial habitat construction, when in fact the reverse is true. This is not to say that artificial habitat construction is that easy either; we still haven't perfected that for really long-term applications. But interstellar travel is really, really hard.

Now, of course, it is possible that we've completely overlooked some fundamental physics and it's actually not that hard to travel light-years very quickly; this would change the equation entirely. There's a Neal Stephenson short story about this, where some warlike and somewhat primitive aliens attack Earth, and are easily defeated by our far-superior weaponry (they basically had 19th-century cannon technology). So the humans get their technology and figure out that faster-than-light travel is actually really easy, we had just overlooked something simple all this time; at the end, the aliens are crying about what they've done, because then the humans set out to become galactic conquerors.