Maybe there's a good reason not to, but wouldn't it be a lot simpler to equip a rover with a microscope to search for living and fossilized microbes?

We should determine everything we can without disrupting the Martian environment.

We don't know enough about life on Earth, let's spend those billions to know more about the environment (massively disrupted by we humans) within which we live, and will have to adapt to, if we even can do so.

Ban spaceflight so the universe remains pristine?