Yes, the modern pre-hung window has the bottom part as an integral part of the assembled window. (They didn't make them like that in your day, Giordano. Good to know hell has the internet, you heretic.) But what if that window is set in a wall that is so thick that the weathered outer sill does not reach the outer face of the wall, let alone project from it slightly as is the custom in modern house construction? What would you call the stone, brick, or even metal shelf that extends beyond the pre-hung window? What would you call the piece of granite I'm looking at that sits upon the top of the brick facing of a thick apartment building wall and extends well beyond the sill but is most definitely part of the "window opening," i. e. the hole in the wall awaiting a window? I say window ledge.
Being a language person and not an architect, I suggest that it is from such forms of construction that the socially constructed distinction originates. Yes, the interior "sill" is not in fact part of the window at all and therefore more of a ledge than a sill, but if everyone calls it a sill and it's made to look aesthetically as if it were, and maybe it once was, before pre-hung windows--(Do you still remember that far back? Do you remember how you died?)--then it's a sill just the same.
But trying to get people to stop calling it a sill would be like trying to get people to stop saying "cement" sidewalk, when it is in fact concrete.
That's how language changes.
Ciao!