I have this idea for a science fiction story (It actually came to me in a dream.). In the distant future, the inhabitants of the Milky Way have formed a galaxy-spanning civilization similar to the Republic in Star Wars. However, instead of using FTL starships to travel between planets in different star systems, they move the planets. Each member world has been outfitted with a powerful jump drive allowing it to travel thousands of light-years in the blink of an eye. It might work by enlarging microscopic primordial wormholes (Einstein-Rosen bridges) and then "warping" through them, but that's just one option.

A planet will use its jump drive to teleport from its current star system to another one, where it will assume an orbit in its habitable zone. A given planet will undergo about 1 to 4 jumps per day, with each jump being planned weeks or months in advance. When two planets are in the same star system, people use sub-light starships to travel between them. After a ship makes the trip from one planet to another, it can land or it can enter low orbit and "ride out" the next jump. Since there may be a dozen or more member worlds in a given star system at a given time, a single ship can reach any other member world within only a few jumps (Think Six Degrees of Kevin Bacon.).

My question is, how could I justify this system? What principles, based in real physics or sci-fi logic, could I use to explain why using a starship for FTL travel is impossible/more difficult/less efficient but using something as massive as a planet to do it is possible/less difficult/more efficient?

Keep in mind that FTL communication exists in this setting, and may use a technology similar to that of the planetary jump drives or something completely different.

Edit: I might call my story "The Planetary Exchange" as a parallel to the Stock Exchange. And the jumps would be coordinated via a central computer network; it wouldn't be a chaotic free-for-all.