Because the car already has multiple redundant safety systems. The bicycle has exactly zero.

Maybe so, but auto accidents still result in lots of head injuries even with the existing safety systems.

You're right, if anyone was rational we'd all wear helmets in the car all the time.

It's all irrationality and a belief that cars are much safer than we pretend they are.

As someone who has worn a helmet in the car from time to time, a lot of cars don't even leave enough headroom available for someone 6' tall to wear the helmet without their head hitting the ceiling.

Is that a reason to adopt no safety systems for a bicycle?

Not sure if you've ever worn a racing/motorcycle helmet but the visibilty/mobility is not the same. People would shoulder check even less than they do now.

I'm talking about a bicycle helmet.

I think that'd be pointless, right?

Not sure I follow you, are you suggesting bicyclists should wear motorcycle helmets?

Why not mandate a HANS device for everyone? Basal skull fracture is no joke and happens often with head-on collisions...

Right - good point. And obviously my larger point is where do you draw the line with mandates? The arguments here are largely about preventing head injuries - so how far is too far?

Given trends started in the 60s, every year we add more and more safety measures so I guess nothing is too far in the future.