Because in some cars your destination is your goal, and in others,the goal is the process of reaching it. Great acceleration is very fun, but you get used to it, however the car that you feel, even if it weighs 2 tons is a whole different experience and it doesnt matter how much you drive the car. For the record i dont think that 6er is a particulary very good drivers car, even in its own segment, but if i had to choose rs7 or 6er, i would pick latter for more driving envolvement and lose couple of milisecs in accelerations. But the biggest worry for me would be suspension of rs7, in couple of tests they say that its uncomfortable and in this kind of car its borderline untolerable.
And pure enjoyment is not a reason to buy 6er gc, but just like pure acceleration cant be the reason to buy rs7