I am wondering why gravity needs to be unified with the other three forces into a super-force. Why can't the three forces be seen as interactions between particles in a space-time fabric whose geometry gives the impression of gravity?
It doesn't need to be unified with the other three forces. It's just that unification has, so far, led to increasingly accurate and arguably more 'elegant' theories. It's entirely possible that gravity simply cannot be unified with the other fundamental forces.