From what I'm led to believe, dragonborn are defined as someone whose ancestor was a dragon and they are as a result 1/3 or so dragon. They consistently take on features that make them look like humanoid dragon people. Then there are half dragons, which are more direct in the humanoid-dragon chassis where their parent was a dragon and the other parent was a humanoid, who are defined by having tails and such where dragonborn do not have.

But dragonborn can't start families with other humanoids, can they? You hear about half-orcs and half-elves, but never a half-dragonborn, it's just not a thing. Some lore points I read indicated that they could only reproduce with other draconic species, which would be themselves and kobolds (and actual dragons probably)

So if they're defined by being partially draconic - but not as much as a half dragon - but can only make families with other draconic individuals/species - then how are they not all half dragons? If they can only be born from draconic entities, they'd have to be born from a dragon, and other dragonborn that are directly related to the dragon that spawned them

Edit: This topic has proved to me that the subreddit for 5e isn't the place to discuss 5e lore. Go on with your day.