Expansions dont come bundled with the previous ones by default, thats a concession made after the expansion is released.
If you want to play an expansion now, then you need to buy that expansion now.
Not sure what's so hard to understand about that.

Otherwise you'd be playing current content now without paying for it, while others who have already purchased Stormblood would still be charged the sane amount to preorder Shadowbringers.

Otherwise, why not preorder expansion 7.0 now and get 5.0 and 6.0 included?