If your solution is 'physiology doesnt matter, they are not real' then its simply not a conversation. In that scenario, yes, they can certainly be whatever you need them to be.
But I never said--and have never said--that physiology is
completely unimportant.
I am simply saying that physiology is not absolutely determinative of what characters are capable of achieving, beyond certain relatively niche things like dragon breath, impossible luck, innate but limited teleportation abilities, flight, etc.
Whereas you are, very explicitly, making the argument that
not having hard-and-fast limitations on what someone can do, simply because their physiology would be less physically adept in our world, causes absolutely all characters to be 100% perfectly identical with no differences
at all. You have made this assertion in an extremely dismissive, non-discursive way ("Humans with funny hats it is.") I am saying that an actual discussion is, in fact, possible here.
To put my money where my mouth is and
demonstrate why I think this is false, I will bring out my classic soapbox of dragonborn. Being
extremely brief: MUCH faster development, better at healing (at least in 4e), different means of dealing with body heat, egg-laying, high-protein diets, elemental halitosis, etc. These factors
should all bear out in their art (painting, poetry, music, etc.), their material artifacts (architecture/civic engineering, clothing, furniture design), their language (metaphors, accents), their food (materials, preparation methods, flavors), etc.
Not one of those things has any meaningful impact on whether a dragonborn can achieve superlative success through mighty thews or shrewd thought or wise words. Yet all of them individually, and certainly when taken collectively, make for a meaningful distinction from humans in general, and any specific human culture one might consider. Dragonborn might not be keen on chairs, for example, if they have tails (classically they do not, but tailed dragonborn art is at least as old as 4e dragonborn if not older)--this would make them more similar to Eastern cultures IRL where one usually reclines on floor-cushions rather than having knee-level frame furniture. Dragonborn wouldn't design prisons with ordinary metal bars as their primary deterrent because, y'know,
elemental halitosis.
Unless there is something
more to the dismissive "humans with funny hats" description, the only thing preventing fantasy races from being distinct from one another is laziness on the part of the people describing them, be it the game's makers or the person running a specific table.