Robots specialized in assembling one machine are not general-purpose robots. I doubt they would be able to "do pretty much everything a human could want to do".

Now, if someone does perfect a general-purpose, AI-driven robot, then yes, sending that robot on the Mars mission instead of a human would make a lot of sense. That's kind of, kind of, what we've done already with the rovers. But until such a general-purpose robot is actually created, human beings are still the most versatile, adaptable entities that could go to Mars.