A fixed starting position allows deeper study (of both opening and middlegame) so the competition is more layered, and the skill required is broader and deeper. And I don't mean memorizing to move 20, I mean e.g. knowing standard pawn structures or middlegame themes. This also adds to the sporting aspect when a player deviates from the norm with not only moves but with ideas.
My opinion is that in comparison 960 is not a complete game.
Give me a good reason that classical chess is better than 960.