I think it wouldn't fit with the lore... according to it, the dragons aren't really liking them. Lunus are rather seeing some of them as slaves, some as threats ; Helians are seeing themselves as higher than the bipeds, suspicious at best, and think they should lead them towards improvement ; ...
Global dragon society has too much pride about that, and the rare dragons who would be ready to be ridden by a biped would be like outside of their society. That is just my point of view, though