A particularly topographically challenged building in Paris has exits to two enclosing streets end up on different floors. The elevator is numbered -2, -1, rez-de-rue [exit north], rez-de-chaussée [exit south], 1, 2, ... .
Don't know French, but I wonder if the meaning of "etage" is similar to Polish "piętro", which literaly means something like "elevation". So, basically in Polish we have a specific word for ground level, and then we count how much elevated above the the ground the current level is. That's why "1st floor" is the one "elevated one level above the ground".
In English you count "floors" and floor is a usable, hard surface on which you can put something, like a chair. That's why a floor on the ground level is treated the same as the floor above it - it is equally good on accommodating chairs, beds, and other stuff.
Etymologically, étage comes from the Greek στέγω (and gave the English word “stage”); it is a typically wooden cover. Since the first floor was often instead a continuation of the outside road (way back!), it was not considered a “stage”.
