There is much more useful mass of material in a planet than even a pile of asteroids... In fact, until you start gathering up the entire Oort Cloud (current best estimates put the entire Oort Cloud as x5 earth masses). From the inner solar system you don't even have as much mass in asteroids and comets to equal the mass of smaller "planets". It's not a stretch to imagine that you want concentrated mass to facilitate efficient industrial scale operations. It takes time and energy to drag lots of small mass together to build a big interstellar ship. So your hypothetical interstellar civilisation might want to break up large rocky planets into smaller "low gravity" chunks for processing, which if your an interstellar civilisation, is probably best done by slamming nice dense metal mass into the planet at high speed to throw big molten chunks of it into orbit, you want larger than dinosaur killer size glancing hits, not planet cracking "the moon hits earth" sized ones. This whole process gets much easier once you strip away the atmosphere as well.