If they really wanted to link VII and X, they should have established that in the design of Spira.

Not that linking them would've been such a great idea. You get to see most of the Planet in FFVII, so there's not many new areas to explore in the sequels. Linking games is easier if you simply explore a part of the world in each game. Like the Ivalice games or the Suikoden series.

EDIT: Takara beat me to it.