I guess, for me, the idea that monsters have "species" makes them less interesting. That kobolds, for example, are just a scaley kind of hominid or whatnot makes them less fantastic.
All comes down to sensawunda I suppose. For some, it's the codification of spells. For me, it's the codification of creatures.
It depends for me. I like some seriously fantastic explanations for seriously fantastic critters (especially the chimerical ones like manticores and, well chimeras). But on the other hand, to me the presence of monstrous races or beasts that effectively have ecologies of their own makes the
world more magical. I find it actually injures my suspension of disbelief if a world has pretty much our own animal and plant species and everything not found on Earth is weird and strange even to the locals. It makes me feel like the ordinary humans aren't really residents of a fantasy world, in a way; they're just Earth-humans who live there. I prefer things like the world of Steven Brust's
Jhereg; a bunch of freakish new wildlife that's considered just as much part of the world as beasts that are familiar to us, with hawk and tiassa right side by side on the Cycle.
All this should be taken alongside the disclaimer that I like owlbears (and that I even like owlbears as perfectly natural giant killer monotremes that are just peculiar to the D&D ecology), of course.
Dragonborn are magically created. They are created by a god as is everything else in a D&D world. Thus, the god in question can give a big one fingered saluete to the entire scientific community. Why? Because he's a god.
Oh, sure. I just find that can kind of undercut the dignity of the god in particular to me. "Shut up, I like boobs! I'll put them on anything I want! You're lucky I'm not putting boobs on beholders and trees and mailboxes. Mmm... boobs..." [/Homer Simpson voice]
I mean, I am aware that there are plenty of mythical gods who have zero sense of dignity. I just kind of hope that Bahamut's not one, you know? I frequently say "a god did it", but optimally I want to make sure a god did it for in-character reasons, making a neat new myth instead of just a handwave. After all, actual myths will always give you more of a story than "a god did it," even if it's "Bahamut lost a bet with Coyote, and Coyote really really likes boobs."