And yet, it turns out there have been various attempts at a motoring helmet over the years:

https://colvilleandersen.medium.com/the-case-for-motorist-he...

So that does open the question about whether suppression of this lifesaving technology has been something intentional on the part of automotive companies and enthusiasts (see: "jaywalking") or if it's the other way, that making road safety issues about only cycling helmets is an attempt to de-legitimize that mode and blame victims.