You could only compare individual pieces if acc didn't exist or could be weighted... But it does and it can't. It makes sense because acc requirements change depending on the fight which changes the corresponding BiS for each player.

That 11 Det is valued the same in any set but what options are available in competition to that 11 Det changes depending on the fight and requisite accuracy. What if dropping that 11 Det means you gain 16 crit and 16 skill speed somewhere else? That's why you can't just compare individual pieces.