If you want the actual D&D reason, it's because divine existences can breed with anything (for a similar reason Celestials and Fiends can breed with anything except each other, because they are mutually incompatible).
Dragons in D&D are actually in a complicated position of all being demidemidemi(etc.)gods. Every single dragon is descended from either Bahamut the Platinum Dragon or Tiamat the Polychromatic Dragon, who themselves are the children of the ancient god Io the Ninefold Dragon (also called Asgorath, the Great Eternal Wheel).
The dragons are much distant from their great ancestors, but they retain the divine spark deep, deep within them, and it's enough to make them universally compatible.
A fun little aside, supremely powerful dragons can activate their divine lineage and achieve godhood under their own power (without acquiring an existing divine portfolio or permission from an overgod like everyone else who wants to become a god).
Dragons are big deals in Dungeons & Dragons. You may guess this from the title.