State reason why space probes are usually unpiloted?

Think about it - space probes are basically satellites that just go out into space for years at a time to take pictures of anything they see.. Taking food and supplies for that amount of time would be extremely expensive, and it would be almost impossible for the rocket to lift off the ground if there's that much weight inside. Besides, if astronauts really want to go into space and research, they can go to the International Space Station. A rocket is small and cramped to be staying in for so long, but the ISS is huge, has a lot of storage space, and labs in which to test things. If you had to choose between the two, where would you go?