KH needed FF to establish itself as a series.
KH has enough of an established, isolated lore that it doesn't need FF characters to thrive.
We have Xehanort, all of his variations, the organization, sea salt trio, Disney trio, destiny island trio and the wayfinder trio.
AND MORE.
KINGDOM hearts without FF characters loses half its charm btw. This is yet another reason why kh3 base game is absolutely shit besides gameplay and story.