Isn't a planet just a big spaceship with things built in? You can dig inside it for things, you don't need to capture asteroids, planets can travel through space and have a protective shield - the atmosphere, and Earth is already traveling at huge speeds, which are at very least on the level with the fastest spaceships we can currently make. Sure guiding a planet maybe harder than a spaceship made specifically for that purpose, but all the benefits of having resources right underneath, and planets are a proven space ships, with billions of years of testing, no man-made spaceship can match that for billions of years.
> planets can travel through space and have a protective shield - the atmosphere
Most worlds in our system don't have a significant atmosphere, most of those that do have too much of it. Like gravity, atmosphere also poses a problem for getting back to space.
> no man-made spaceship can match that for billions of years.
Not unmaintained, no, but if it was unmaintained that probably means everyone who lives there is dead anyway.
