Robots can't reproduce. They can't repair themselves. They can't even fix each other. One day in the distant future we'll build robots that can do those things, I'm certain of it, and then we can build planet-colonizing robot swarms. But even then, the most useful thing they will be able to do is prepare for the arrival of humans. Robots won't be ready to replace humans for the purposes of science and exploration until they're ready to replace us altogether, which is a long way off yet.