Which is my point. The DM reserves for themselves the right to decide at any given time if a given creature exists in the world or not, but somehow the same courtesy isn't reserved for PC options, despite the fact both can exist in the same Schrodinger's Monster Manual.
Put another way: If I asked all of you if Xvarts exist in your world right now, there are quite a few of you who could answer yes or no, but there would a sizable number who answer "maybe, IDK, the PCs have never encountered one". Yet that doesn't apply to dragonborn. Most everyone here has made up their mind, and very few of you would answer "IDK". Why? Well, I'm guessing the answer isn't setting integrity/consistency, theme/tonal consistency, or any other philosophical argument. It's purely because the DM doesn't HAVE to play those cards face up (unlike PC options) so they choose to ignore them until there is need.
Which does beg the question: if Xvarts can potentially exist just off camera until the DM wants them, why can't dragonborn exist just off camera until a PC wants them?