Because car or no car the head still goes crack when it hits the road. To me it seems obvious to reduce a risk of fatal injury with something as simple as a helmet.
If you've decided that the risk of head injury when you walk somewhere is so little that you'll take the risk and not wear the helmet, you've made the exact same decision as the cyclists. In the Netherlands, because they have a sensible traffic system, the risk has been reduced so far that many, many people make that same decision that you do when you walk without wearing a helmet.
And I've experienced that a cycling helmet is good to wear. There was this child that appeared from a bush to the bike path. I hit the brakes and went over the bar. A slight bumb in the head but no injury there; broke a radix bone though.
Please do not be offended, and by all means keep on wearing a helmet if you want (after all it is safer, no denying that) but maybe there is room for improvement in your cycling? Those two things I quoted seem to indicate you haven't completely mastered cycling, or your bike isn't properly setup, or a combination of both. I see them for instance as well with foreigners who never owned a bycicle and then come to a country and are given one. For an experienced byciclist riding is usually a second nature, like walking, and falls don't happen more often then they do with walking. Note I'm talking 'normal' cycling here, not for instance downhill or bmx where not ever falling sometimes means you're not pushing yourself hard enough :]
And since bikes don't have ABS, I either have brakes that are not powerful enough, or I have brakes that throw me over the bar if I a mistake in a surprising situation (such as in the case I mentioned). Most falls I have are due to ice. Every winter morning is a decision: do I take the bike with studded tyres, or the one that is much lighter to ride? Sometimes I make the wrong assessment.
Other people made the decision that they're happy with the risk of not wearing a helmet when cycling.
What is so special about you? Are you some kind of super-powered genius, and your assessment of personal risk is some kind of magically "absolutely correct" answer? Why should your personal choice about acceptable risk be made law? (Trick question; it shouldn't)
This happens in two ways: car drivers drive more dangerously when they see a helmet; less people ride a bycicle and so there is more pollution and more deaths or more lack-of-exercise related death.
In the U.S. and U.K.. sadly this is true. ( http://news.bbc.co.uk/2/hi/uk_news/england/somerset/5334208.... )
"Dr Walker thinks the reason drivers give less room to cyclists wearing helmets is because they see them as 'Lycra-clad street warriors' and believe they are more predictable than those without."
"To test another theory, Dr Walker donned a long wig to see whether there was any difference in passing distance when drivers thought they were overtaking what appeared to be a female cyclist.
"While wearing the wig, drivers gave him an average of 14cm more space when passing."
However, allowing this to remain true is victim blaming. A driver should be forced to respect a cyclist and give them room.
In California, (yeah California) there is now a 3 foot minimum passing distance ( https://calbike.org/advocacy/giveme3/give-me-3-faq/ )
"The law is invaluable when a bicyclist is injured in a collision. It establishes a clear basis for citing motorists for unsafe passing."
"How do motorists give three feet on narrow roads?
"State law has never guaranteed motorists a right to pass whenever or wherever they want. Motorists may only pass when it’s safe to do so. This doesn’t change with the Three Feet for Safety Act.
"The new law requires motorists to slow down and wait to pass only when it is safe to do so. Motorists have to be prepared to demonstrate that three feet were NOT available and the slower, closer pass was done according to the law. This is a higher burden of proof for drivers than we have under the current law, which places no conditions on how to pass at a “safe distance.”
So hopefully, the police will aggressively ticket motorists acting in a way that is unsafe for cyclists.
Of course safety is important. Not having lights on your bike in winter is very dangerous (drivers can't see you in their mirrors) and the police do frequent random stops and issue fines.
Anecdotally a friend of mine was in a very bad cycling accident (hit by a bus). She had no helmet and it wouldn't have helped. After months of rehab for her broken leg she is cycling again without a helmet. Of course she could have fallen differently and a helmet would have helped, but most of the damage was caused by the collision and not the fall.