[cthalupa]

If you're a spacefaring civilization, you probably don't live on planets to begin with. Terraforming is hugely energy inefficient, planets that fit the conditions to be comfortable for you biologically are probably rare, etc. Even ideal planets are incredibly inefficient when it comes to use of material.

You can support many times more people in things like O'Neil cylinders, bishop rings, etc. than you can on planets, using less resources. If you're the kind of civilization that can settle a galaxy, you're also advanced enough that you're going to care about that sort of thing, because you're already working on huge timelines, and you probably want to stretch your existence out for as long as possible. Being efficient with resources is going to be important there - the universe isn't making any more of them.