Dragons lair inside mountains where radiation is naturally higher. Gold is an amazing shield against radiation as its about as dense as lead (but far less toxic too) and actually has the fewest naturally occurring radioisotopes of pretty much any element as it is more than 99% stable. Cancer formation increases with age of any living organism (roughly 1% per year in advanced organisms) so sleeping on a bed of gold helps drastically cut down on stress and radiation exposure, two of the most direct causes of cancer until they mature to the point they learn powerful spells like cure disease to deal with it for good.
Or because shiny things are cool. One of the two.