Unlike cycle helmets, I haven't noticed any 'motorist' helmets on sale in Halfords, Wilkos, etc. Plus, it's easy for the non-cyclist to imagine us cyclists falling off left right and centre, just as they imagine they would should they straddle a bicycle.
It's been tried but didn't really get critical mass. All we need is the AA, RAC and some well-meaning fools at safety charities to get behind this like certain foolish cycling organisations and safety charities did bicycle helmets, then persuade some retailer to stock them, police and government to recommend them and so on, and sponsor some motor racing drivers and others to go on TV saying how they would never drive any car without one:
Or alternatively, we could try doing stuff that actually reduces road casualties. I think a problem is that cyclists are just too blasted nice and not willing to make motoring horrible just to reduce the numbers of motorists rather than reduce casualty rates.