Are you aware of how heavy duty some of these parts are that go onto rig??? These rigs are put under a ton of stress drilling 1000's of feet into the ground.
Some of these parts can take many weeks to manufacturer with tight Q/A controls.
If the manufacturer were to ship a rig that was defective, it would cost bucko $$$$$ to send employees and machinery to get it fixed. This is why they wont "cannibalize" a rig. They will ship it when its PERFECT.
My posting contains many opinions. So please do your own research and validation.