I think there's a third, bigger reason: FIDE rating is based on classical time controls. Nobody uses classical time controls online for various reasons. Even

I think there's a third, bigger reason: FIDE rating is based on classical time controls. Nobody uses classical time controls online for various reasons. Even rapid is a somewhat different skill set than classical. Of course these things are correlated, but e.g. there have been times in the world championship when one of the players thought about a position for 20+ minutes before making a move. That's hard to do, and requires very strong visualization skills.