"Why can't we equate the software inside a computer inside a vehicle with the mechanical equivalent? What then would our rights be?"
Well, probably because software is unlike the mechanical equivalent. How could it not be? When I own a hammer, and I sell it to you and hand it to you, you have it and I don't. Music scores, software, movies etc. don't work that way, so they need a different system to work in. That system is based on contracts, because we already had those.
No legal or moral principle mandates we treat all software identically under the law without respect to use case, though. Tractor firmware, Adobe Photoshop and that digital copy of the U2 album you didn't want are all software, but they're expressions of very different things with distinguishing characteristics. In the case of the tractor firmware, an obvious distinguishing characteristic is that the firmware is just as critical to the function of the tractor as any mechanical part. I don't see any reason why the law couldn't mandate that you own your copy of that firmware. (Actually, I don't see any reason why the law couldn't mandate that in all the cases I mentioned; I think people on both sides of the IP debate are making a categorical error by assuming that the zero marginal cost of software reproduction changes everything. It doesn't. It just changes, well, the marginal cost.)
In another comment, I think you said that you thought describing this as "challenging the notion of ownership" as pandering to the (presumably anti-IP) audience. Maybe, but there's very little difference in practice between "you do not actually own your tractor" and "okay, you own your tractor, but don't own this critical thing without which the tractor cannot run, and you're not allowed to modify that thing in any way or even replace that thing with something you can modify." The second clause doesn't give whoever owns that critical thing possession of your tractor, but it gives them at least some level of control of your tractor. Maybe you're comfortable with that, maybe you're not -- objectively the same case can be made for my iPhone, after all, and I'm generally okay with that. But if my iPhone is put out of commission for a week by a firmware cockup I'm just going to be inconvenienced, whereas a farmer might lose a lot of potential income if that comes at the wrong time. I don't think it's irrational to argue that the law should recognize that difference.
Well, probably because software is unlike the mechanical equivalent. How could it not be? When I own a hammer, and I sell it to you and hand it to you, you have it and I don't. Music scores, software, movies etc. don't work that way, so they need a different system to work in. That system is based on contracts, because we already had those.