I strongly doubt this is the main reason. I think it's simpler and just like most hardware: there's no perception that open source adds value, and re-negotiating IP agreements with hundreds of sub-vendors would be unreasonably expensive in and of itself even if the vendors were amenable to open source. We see the same thing in plenty of non-safety critical hardware areas: board support packages, device drivers, graphics stacks, and so on. There's no perception that open source adds value in the hardware industry at large.
The real shocker would be how incredibly crappy it all is. But yes, licensing is a huge part of the problem. But that also happens to help the manufacturers who really wouldn't want to open this stuff up to scrutiny anyway.
If only because there might be a significant number of findings about accidents that turned out to have been caused by a malfunctioning vehicle after all when right now these are attributed to the driver.
