Why the fuck not?
Not in the same cup at the same time she means I think.
Why the fuck not?
Because of the taste, fearing the taste, or out of habit.
I once ate apple with coffee, and found it so foul that I never tried it again.
But, I do like apple-pie with coffee.
So, who knows, apple-juice mixed with coffee may be delicious.
I'm just too chickened to try.