Why do guina pigs shed?!?

ok i went to petco a couple days ago i was thinking of getting a guinea pig and i asked one of the people that worked to to let me hold one to c what they would do and when i was petting him n playing with da piggy i realized they were way better than hamsters! so he let me hold it and when i went to put it back in the cage their was a WHOLE bunch of fur on me why do day shed so much?