But what makes it intangible? The really cheap (but still non-zero) cost to copy? In which case, why would this not apply to any physical machine that has a r

But what makes it intangible? The really cheap (but still non-zero) cost to copy? In which case, why would this not apply to any physical machine that has a really cheap cost to copy? It seems the answer is 'Because it doesn't.' at which point I see copyright law needing to be overhauled. For example, why can my painting be protected by copyright but my engine design not? And if they are both protected by copyright, then why can't I just give a license to use the engine design while selling them the actual physical material (N kilos of iron, M kilos of carbon, etc.)?