This is an interesting juxtaposition, but I’m not (really) sure it’s true. Is a rocket, even one with an advanced control system such as those designed by SpaceX, really ‘simpler’ than an electric vehicle? We certainly think in those terms, because rockets are exotic to our experience whereas cars are experientially mundane for most of us, but I don’t think the facts of the matter are that clear cut.
They're certainly more complex as a result of higher quality demands. A car can be tested under real-world conditions and fixed if it shows any manufacturing defects. Any mistakes with a rocket are very expensive.
Testing is also very different. For a car, you can build a batch and test it extensively under all conditions, fixing errors as they come up. If you develop an assistance system, you can test this extensively before releasing it. Rockets are so expensive that SpaceX had only a few trials for their landing system. You need to make sure that both hardware and software work perfectly without ever testing them in production.
It's worth noting that one big advantage SpaceX has over other rocket makers is that SpaceX gets to inspect landed first stages. That makes their rocket development closer to airplane development than everyone else's rocket development, and no, it doesn't have to be perfect before it is tested. It just has to be good enough to land.
There was a point in the early 2000s (and I don't know how long it persisted for) when ESA had a very hard time hiring rocket scientists because they were all being slurped up by car manufacturers to work on drive-by-wire systems.
And how does that compare to the development of early combustion or electric engines?
I'm not so sure about electric cars, but I would judge a rocket to be less complex than an ICE car. Rockets just have much more spectacular failure modes and are more expensive due to size.
