Because they are rodents.
Guinea pigs are the best animals in the world. They are cute and friendly, and although they poop everywhere, their personality makes up for it. Guinea pigs are actually very smart. They can learn to recognise different people's voices, and can learn when to hide wheel they are a pet. Also, how could you not love the cuteness of their smiles and the way they scream at you when you don't bring them lettuce? It is the cutest thing you will every see.
They do poop everywhere, I agree. But they are SO ADORABLE