Building even basic things is difficult. Friend of mine tried like dozen companies from around the world to manufacture very basic milled aluminium part. None of them could make it. Only one company could make 1 or 2 right out of 100.
Then parts have to be hard anodised and sand blasted.
- Sand blasting uneven - Sharp edges remaining - Streaks - Contamination - Exposed bare metal (it is only acceptable in the holes that could be used for hanging)
Companies also have virtually non existent QA. You can list them things that will cause rejection and they send it anyway.
One company told him after sending a batch that was complete carnage "We can give you $100 coupon for next order!" he insisted on rework. So they sent again, even worse.