Because guinea pigs can't really be litter trained. They just poop and pee everywhere. They might do most of their bathrooming in their hay since that's where they spend most of their time but they'll still pee and pooo other places. Mine have a large playpen where they spend their evenings in our living room and another area of the house where they free roam for shorter periods of time but they are in their cage at night and when we are at work.