Insofar as I remember the setting novels, this is very incorrect, given that Kingpriest of Istar was an arch-blasphemer to an unbelievable extent, who not only lost/never had clerical power from Good deities, but whose policies resulted in disappearance of genuine clerics, the fact that he deliberately covered used magic, and who managed to make every single god agree that he and his whole nation need smiting.WiserOdin032402 wrote: There's the Gods of Light from Dragonlance (Thank you for the reviews, Libertad, otherwise I wouldn't have that example) who backed the Kingpriest of Istar and allowed him to try to wipe out neutral-aligned faiths, try to wipe out arcane magic, and commit genocide.
Returning to the main question. There are two reasons, in order of importance:
(1)Writing characters who objectively have a lot of power and do not act like self-righteous jerks is hard. I can name enough best-selling writers who cannot think of anything better than "antagonists are bigger assholes/have committed a laundry list of evil deeds, therefore our protag's behavior is OK". (Meanwhile it is safe to say, that no one with a chance to make it as a bestseller writer would waste time on writing RPG products.) Writing characters who objectively have a lot of power and must NOT use it most of the time if you want to have an actual story is harder. Writing characters who have not just a ton of power but also supernal knowledge, whether due to general superintelligence or defying causality, well, you get the idea.
(2)DnD was born by making a kitchen sink crossover of several fantasy universes with incompatible (and often vague/poorly thought-out) metaphysical assumptions, its moral framework was then by necessity heavily skewed towards justifying its murderhobo gameplay, its cosmologies have varied over editions, but tend towards clumsily veiled total grimdark, so basically who the fuck knows how Big Goods are even supposed to act in such a setting? Authors have come up with their own idea, and that takes us back to #1.