Used to be. I think this is either the actual solution, or it is at least going to be mine.
Before, Elves could change shape, their form was magical. After the whole fight between the Elven Gods, that ability was taken away.
But it didnt happen immediatly. As the Elven diaspora moved out into the multiverse, they adapted, magically, to their new environments.
And that is why Sea Elves, are biologically distinct from Wood Elves, are biologically distinct from Astral Elves, are biologically distinct from Drow.
Because Elves were once magical, and as they went out to the multiverse, they adapted, before that ability was lost.
#ScribeCanon.