I don't understand Game Freak at all. What benefit is there in preventing players from switching off the Exp. Share? There is none!
I can at the very least imagine why GF removes and reintroduces other features every gen, it's so they don't eventually become stale. It's a shitty reason but I can still see that being a thing they would do. But there is literally no downside for GF or the player in making the Exp. Share optional.