I think what really needs explaining here is what is meant by "perpetual motion". If the meaning of the phrase exactly matched according to the definitions of the individual words, then perpetual motion would be the most common thing in the universe and include electrons orbiting nuclei and planets orbiting stars. These things will happen "perpetually", unless you try do one thing which the more common meaning of the phrase requests - extract energy. An electron orbiting a nucleus both contains an amount of potential energy, and, since it orbits in a perfect vacuum, does not expend any of it's energy in it's perpetual "falling" around the nucleus. Planets orbiting stars are almost as perpetual, but due to the not-quite-perfect vacuum of space, do actually give up some of their potential energy continuously when colliding with atoms in space.
Remember that objects under the influence of fields, just because the may be moving in orbits, are not necessarily preforming work. It is the same as a mass moving through interstellar space in a straight line. The mass is moving, but is not performing work unless it collides with another object, and in which case, energy will be conserved, but not created (or destroyed, for that matter), in the altered trajectories of the 2 objects.
The goal of "perpetual motion" as more commonly used is to be able to extract energy from a device without affecting the potential or kinetic energy within it. Clearly impossible. The only apparently possible way to do this is to somehow build a device that extracts energy from an alternate universe, yet even then, doing so will lessen the energy from the alternate universe. It would be much simpler to extract energy from more conventional means.