A fairly easy explanation for it would be that "dragonborn" are actually humanoids; they're like "fiendish" or "celestial" creatures with heavy influence from long-distant dragon and half-dragon ancestors, but generations of interbreeding and more human-breeding (including human-based selection-for-attractive-to-human-mates) has led to their current form.

Since human males like their mates to have sexy curves, dragonborn males have inherited this preference, and dragonborn females with curves have been preferred mating partners for both dragonborn and other humanoid males.

Similarly, humanoid females select their mates for certain criteria that have led to dragonborn females inheriting these preferences, and dragonborn males being selectively bred to meet them.


Essentially, even if Dragonborn don't use certain secondary sexual characteristics for genuine survival benefits, their humanoid ancestry creates sexual-selection pressures which make them have those traits.

More basically: they're sexy because humans find them so. And not just in a meta-game sense.