White dwarfs, despite their seemingly peaceful nature, are actually the result of some seriously violent star death. Although we're not talking about the explosive ferocity of a supernova (these stars are too small to ignite a supernova), when stars like our sun enter the red giant phase, their surfaces balloon-up engulfing any nearby planets.
For example, it is predicted that our sun will swell out to beyond the orbit of Earth when it dies in 4-5 billion years time. While a swelling sun, expanding to fill our sky, might make for an impressive sight for anyone (or anything) living on Earth in the 40 millionth Century A.D., Earth will be toast.
It is also predicted that as the sun violently sheds its outer layers, vast quantities of mass will be hurled into space, causing the remaining planets to spiral deeper into space, away from the dying sun.
This is the key reason why white dwarfs are often alone, living for billions of years without any signs of a planetary system - the planets are either lost, destroyed or promoted to orbits many times further away from the star.