They might not be planets but planet-sized, artificially-created cities/machinery/engineered objects. Think about it: If you were tasked with creating a planet-sized thing why would you give it anything less than a perfect orbit? You'd also put it in orbit around a super stable dwarf star to maximize the useful life of the project.
Even if they are planets, imperfect orbits lead to problems in the looooooong term. An alien civilization may forcibly alter the orbits of the planets in their solar system in order to stabilize it. E.g. before turning their star into an engine that moves everything along with it across the universe (e.g. out of the way of an incoming problem like a black hole or to prevent an astronomical collision).
Makes me think of how the Puppeteers in Niven's Known Space escaped the explosion at the center of the galaxy by moving 5 planets in this configuration
Even if they are planets, imperfect orbits lead to problems in the looooooong term. An alien civilization may forcibly alter the orbits of the planets in their solar system in order to stabilize it. E.g. before turning their star into an engine that moves everything along with it across the universe (e.g. out of the way of an incoming problem like a black hole or to prevent an astronomical collision).