Why do guinea pigs poop so much?

They're eating all the time. They should eat hay throughout the day, and their GI systems are always working. They have to be pooping all the time because their intestines have to be going in order for them to absorb the nutrients. So constant eating, constant pooping, and constant moving are the way that their GI systems work and the way that it keeps them healthy. What goes in has to come out.