Humans can get a lot more work done, just by their dexterity, flexibility and ability to learn and adapt. For robots, even simple tasks have to be pre-planned, in this case sometimes years in advance.

It is conceivable that you could build a robot/machine that can collect one type of sample and do one type of analysis cheaper and quicker than a Human. However, human astronauts can do multiple types of these, maybe even adapt them on the fly.