I can certainly agree that humans, regarded as perfected creatures of biological engineering, would make for an extraordinary Mars rover. You can make the case that we even the best among all animals for that job here on Earth (we are that good, a fascinating convo for another thread).

The trouble is space itself is really rough in new and different ways. Even if everything is going right, the radiation is extremely dangerous, both on the journey and on Mars itself. And there's bone decalcification which happens very fast. And life support systems issues become very quickly entangled with all the other engineering issues that can cause cascading failures between systems, so even if you didn't think of (say) engineering failures of how power gets to some component as a life support issue, it can become one due to the interdependence of systems.