Except that it's not. I deal with these systems daily. The soda that comes from the keg is 100% drinkable as is, though a bit flat as that is handled by the carbonator on the fountain.
We now have a BIB system, if I were to take a 5-gallon bag and dump it in to a measuring bucket it would not be 5 gallons. A 5 gallon bag will fill about 35 32oz cups with a normal amount of ice in them. I have witnessed this first hand on days where we sold through a large amount of a single product.
32oz is the most commonly sold beverage size in stores, figure that there is roughly 8-10oz worth of ice in a normal cup, some people do more and some people do less of course but 8-10oz on average. This leaves about 22oz for soft drink and there is usually an ounce or two of space at the top of the cup. So figuring an average serving of 20oz (which also happens to be why the price of a 32oz fountain is the same as a 20oz bottle) you would get exactly 32 cups.
The reason these don't get changed constantly is, not everyone buys 32oz cups and there is more than one option at the fountain. We do change our most popular BIBs (Coke, Diet Coke, Diet Pepsi) just about every day.