There is a huge difference here. Cars clock billions of hours of operation in an uncontrolled environment. These must deal with decades of use, abuse, neglect, sabotage, third party parts, etc. Anything that can happen WILL happen in a car.
Compared to this, rockets are tightly controlled environment and running for a very short time. There is also incredible amount of redundancy and supervision in a rocket but you will not be able to have double of every critical component in a car and then have separate controller to check if both are returning matching results and which might be faulty and you will not have a team of specialists obsessing every time something did not work exactly right.
Why wouldn't it be possible to have a similar system in a car as in a rocket? The manufacturer could easily rack up 3 somewhat cheap compute nodes to validate the outputs. This really shouldn't be much more than $10k extra - for the end consumer. And the fact that a lot of cars sell for $100k or more this cost should be negligible for self-driving capabilities.
Every production car is built down to a price, even expensive luxury cars with large margins. All car segments are extremely competitive and any efficiency you can gain over your competitors can be put into more features, higher quality, lower price, or more profit. Over-engineering isn't that much of a marketing point or Saab would still be around.
That said, mass produced electronics are cheap - even automotive grade ones. It wouldn't add that much to the BOM, it would be mostly design cost. I just can't see the justification without it being required by regulation.
Compared to this, rockets are tightly controlled environment and running for a very short time. There is also incredible amount of redundancy and supervision in a rocket but you will not be able to have double of every critical component in a car and then have separate controller to check if both are returning matching results and which might be faulty and you will not have a team of specialists obsessing every time something did not work exactly right.