That may be true in the real world, but this is a fantasy world where the metaphysical reality is quite different from ours. Likewise, your assertion is also not true in a fantasy world like Tolkien's: e.g., Aragorn.
As I said before, there are gods in the multiverse who have the portfolio of nobility, suggesting that there may be a metaphysical essence or reality to "nobility" in the same way that there is for "evil," "good," or "chaos." You want to extrapolate what a "realistic" world would be like for your world-building? Then maybe start doing it by looking at the deeper implications of the D&D multiverse and its metaphysical reality! A god of nobility may even be pushing things behind the scenes to help that audience happen and reinforce the lawful structures of nobility in the cosmos.