The robotic probe is backed up by thousands of scientists, analyzing the data it sends back home, while the man with the shovel is unlikely to even survive long enough on Mars (if he even gets there alive) to discover anything the robot missed.
If I send a probe, I can pack the entire payload with scientific instruments. If I send a man with a shovel, I have to pack food, water, air, habitats, spacesuits, and other tidbits instead, just to keep the man with the shovel alive (for a time).
If I send a probe, I can pack the entire payload with scientific instruments. If I send a man with a shovel, I have to pack food, water, air, habitats, spacesuits, and other tidbits instead, just to keep the man with the shovel alive (for a time).