No one's consuming liters of grapes every day to survive - it's a nice-to-have and not a staple. Transporting it in massive quantities (like enough to make a dent in how much Cape Town consumes every day) changes the equation somewhat, making it prohibitively expensive to do (compared to options like, hoping for rain). Of course, this also changes the equation if we do get to the point that many lives imminently depend on it - people don't riot to be first in line to get grapes.