Steve can probably speak better to this than I can, but street cars don't have a cage. In a race car, the cage is what protects you.

Your shoulder belts are attached to the cage. If you attach the seat (and belts) to the floor then it's possible for the seat to move independent of the cage, which seems bad to me given the shoulder belts are always attached to the cage. Attaching the seat to a frame that is attached to the cage keeps the seat in position with the cage and allows everything to move together.

David