imsmooth:
True, but in order to get to the mold stage one needs to make a final prototype. I've checked and this can be expensive. For the cost of getting someone to do it I can almost buy my own printer. Once I have a good prototype I can have a mold made for production runs.
You would use a service like http://www.shapeways.com . They are not that expensive.
Even if you already did have a printer and that part didn't cost you anything, I think it would still be more expensive in material to print several thousand units than have them injection moulded.
Then you have thousands of hours of print time. Those cost too.
Edit:
All this being said. There could be alternative ways. Perhaps you don't need a custom case? Can you get by with something already existing? There are project boxes of countless shapes and sizes made of various materials, so unless your product absolutely can't be boxy and needs protruding parts, through-holes or similar features, using existing, standard enclosures might be the way to go.