Our family spends a week each year on Lake Powell, AZ/UT. This is an extremely dry climate. You can camp on the lake shore or use a houseboat. When camping, people used to just walk up above the water line, dig a hole and do their duty. They would also bury their paper along with their poo.
The problem is, the high water line is not just above where you are camping, but when the lake is at full pool or 3,700 ft. Well, if the water is low, like the last few years, you will never be able to get above that line and will be burying your poo below the water line. When the water is low people buried figuring it was okay since the were "above the water line". A few YEARS later, when the water would inevitably rise, they found the paper and poo would just come to the surface and float away. There was virtually no decomposition. Now, if you camp on the lake, you are required to take your own port a potty to take care of your poo. They also have pump out stations all over the lake to help keep things clean.
As for Whitney, if they did not require people to pack it out it would be a total mess up there. You can't believe how many people are there on any given day from May to September. It's not fun to take a Wag Bag but it is necessary.