But you can. The cost is higher, but you can make a copy. So if the cost is really small (not $0, but small) it changes? Also to note, in these cases, we are talking about both the code and the rest of the machine. Is this to say that if any part of the machine can be copied for close to $0, we can remove ownership? Any single piece of that engine can be easily and cheaply copied, so once again, why the difference?
The laws of the physical world mean that, if you give/sell an object to another party, you no longer have access to it yourself. This is where concepts like the first sale doctrine come into play - it's your one tangible thing, and you can do with it whatever you like. You control that one, and only that one, instance of the thing.
Intangibles can be given/sold to another party without relinquishing your use of the original (because copies are free). You can buy one CD and gives copies of the music on it to 1,000,000 people without cost, and without giving up your right to the original. Even if you could copy an engine for relatively cheaply, the same economics do not apply.
Therefore, an exception exists to our physical property laws to cover this edge case: copyright law. As userbinator notes, there are other areas of intellectual property law that may apply to physical goods (e.g. patents), but you asked what the difference is between digital programming and mechanical: the difference is that digital programming is intangible, so it can be copied ad naseum, so the ownership exception we call copyright law applies.
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.)?
