If you think a triangulated 4 link and an MII front suspension is "modern" and a major inprovement over torsion bars and leafs you'd be wrong. There is more than just weight that makes a unit body better than a body on frame, structural strength and body stiffness come to mind. The body on frame setup is strong in two dimensions, allowing more flex. Unit body vehicles are stronger in all three, resisting flex better. Even your race car example is more like a unit body in terms of dimensional strength than it is a body on frame.

But hey, it's your $13k not mine.

They say there are no such thing as a stupid question.
They say there is always the exception that proves the rule.
Don't be the exception.