Why are my guinea pigs so shy when they are out of their cage?

i know its good to give them time out of their cage but everytime i put them on the ground to run around, their so scared, i put tunnels on the ground and blankets and they straight away hide under, they dont do this when they are in their cage, when they are on the floor they run in my lap or up my sleeve they look more scared than having fun, is it bad if don't give them floor time? i still let them on my bed and run around and they are fine with that, every second day i let them outside in their hutch and they are fine, its only when they are on carpet or tiles in my house, why are they so scared of everything else but their hutch cage & my bed?