Those of you talking about space are conflating multiple issues. The main reason most space craft are unmanned are their mission types and requirements. It should be obvious why satellites are unmanned. Space probes are unmanned as their design and objective precludes life support and return.
However, the main reason for unmanned craft is weight! Given the cost per kilo to orbit (around $1,000,000 I believe), all that weight spent on life support systems, seats, food and all the other support stuff us poor old carbon units require is incredibly expensive to get to orbit, and thus is sacrificed if it is not utterly needed.