So it has to be an all-or-nothing?
Fine. Then why does the game not have a rule for every single thing a PC is or can do?
See...? Going to the extreme result in an effort to make a point is ridiculous. I can go there just like you can.
There are some fiction that are represented by rules. But not all fiction is represented by rules. And the fiction that isn't represented by rules can instead just be declared as part of the fiction. Who and what those pieces of the fiction are can be whatever the designers wish it to be, as well as whatever an individual player decides they need (and are free to add or subtract from their game.)
It is not required that a species have rules to be considered a species within the fiction and within the game. It certainly can (and oftentimes might be)... but doesn't have to be. A person can play a Gold Dwarf or a Shield Dwarf still in D&D if they decide they want to, even though there are no longer rules to distinguish the two species. The player just identifies their character as such.
The designers/publishers of this game will include those rules they feel they want to include. If anyone needs more rules than that... they are free to make up or find new rules wherever they want and insert them into their game.