These sort of statements are only half true, or else Aegon wouldn't have bothered with being annoited by the High Septon or Jaehaerys with having Barth write the Doctrine of exceptionalism and then having his hired septons travel across the continent to convince people of its truth rather then just telling people to sit and spin when they complained about his marriage with Alyssane. If anything I'd say it's something of a point in GRRM's writing that dragons are an effective tool for conquest, but not actually sufficient by themselves for rule, with the most extreme example of it being Maegor, who ruled by dragon more then any other Targ, and in the end just got assassinated when he wasn't on Balerion because the people he ruled hated his guts. Same with Hugh Hammer and Ulf White who were killed when they weren't mounting their dragons, the former by a Swordsman, the latter by being poisoned.
GRRM even touches about how dragons are tools for destruction, but not necessarily rule, himself:
