The issue with a person doing a week of science on Mars is keeping them alive for the journey there, healthy enough (and alive) during the ground mission, and getting them back alive. Even if your astronaut is interested in a suicide mission that's still the trip there and ground mission. You also can't just send one, NASA figured out three is the minimum for best effectiveness.

So before any scientific equipment you've got literal tons of life support to get to Mars. For robotic probes they can be mostly scientific instruments. They also aren't really limited by consumables like astronauts.

It's not like astronauts wouldn't be cool or useful but for the same cost as one manned mission we can spam Mars with robotic probes. There's also the added benefit of an unexpected lithobraking not leading to some flag draped coffins and a total halt to the program for years.