These planets contain liquid hydrogen in their interior, while the earth has liquid iron in it.
When liquid hydrogen is in tremendous pressure enviroments, the electrons that make up each atom of this element end up "jumping" to other atoms. These "jumps" allow liquid hydrogen to behave like a metal.
In addition, with the constant energy released by the nucleus of planets like Jupiter and Saturn, as well as their rotations, the liquid hydrogen receives induction of currents, giving rise to extremely powerful magnetic fields.
