Because maintainance (and general trouble-shooting) is by it's very nature not something you can predict ahead of time. In fact a lot of the time, it's the kind of work that noone thought would be needed when they were designing the machine you're maintaining. Basicly, you need a highly flexible general-purpose machine for maintaining other machines - and the best one we have is the mark one human. True - so long as you don't actually do anything. If you start mining however, it's a different kettle of fish. The environment becomes dynamic in a hurry when you're shovelling tons of material around and processing it. Indeed - and that's the main reason for Mission Specialists today and the main reason why Jack Schmidt went to the moon on Apollo 17. You move the expertise to the site where at all possible. Because you can perform maintainance. To keep it ridiculously simple, an astronaut can take a rag outside and wipe off a solar panel - a robotic arm through several minutes of time delay risks putting a rag through that solar panel, if it hasn't run out of power by the time the storm blows over. Try putting a rover on the lunar surface for longer than a lunar day then. You'll find that the thermal stresses reduce design life dramatically (and since rovers work slower than humans by an enormous amount, that's a serious problem). Remember, you're talking here about developing highly complex technologies to do what to humans would be a simple task. For example - any manned habitat on the lunar surface is going to be buried in regolith (intentionally) for shielding and thermal insulation. How do you do that? Well, you can give an astronaut a shovel (or a small version of a JCB) and he can do it in a day or two, or you can design and build a tele-operated, semi-autonomous digging robot and hope it doesn't suffer a malfunction and accidentally hole the habitat in the several months it takes to dig the trench, lower the habitat and cover it again. Or that the robot doesn't get fried by the thermal stresses of several lunar day/night cycles. And if you're thinking of multiple robots, remember we haven't figured out how to do that right just yet either. I know, it's not a perfect example, but it's rather illustrative. Microgravity and lunar gravity though, are different things - and we have no idea what lunar gravity does in the long run. And we do need to know, unless you think we can stay on this planet forever... Actually, that's not a dodge - maintainance is what humans excell at. What specific machine are you proposing I cite a maintainance example for?