I don't want to discuss if helmets actually have any safety advantage (there are studies claiming both things).

But I don't think that biking is a particularly dangerous activity. I don't wear a helmet while walking on the street or while driving a car, so why should I wear a helmet when biking?

In a car you have airbags, which are sort of like an external helmet.

As for walking, there are a lot more things that can go wrong on a bike. You can hit a pothole, get your wheel caught in a train track, slip on an oil patch, get doored, have your chain get stuck, not be able to clip out of your pedal, get something caught in your spokes, take a corner on some sand, hit the ground with your pedal, etc.

These things aren't very likely, but any of them provides a fair chance to lose control of your bike and then hit something or the ground at a decent speed. The more traffic you ride in the more likely something will happen. And if you ride every day to commute, you'll almost certainly have some kind of incident sooner or later.

Personal anecdote section:

I bike commuted for 2.5 years in the south bay without any major incidents. Later, I had been bike commuting in SF for about a month when I had an accident that involved hitting my head on a parked truck and then the ground (at least I think). The helmet ended up slightly scraped up. I don't know how badly my head would have been injured without it; it may have been just a scrape as well. But I'm quite happy I didn't get to find out.

It's my observation that blows to the head are more likely on a bike than in either of those situations for two reasons - the speed and the position. When you're biking, you're going much faster than you would walk (I commute every day and often hit 40 mph). Second, in an accident, you often to go straight over the handlebars - the bike stops but you don't. That means head impacts are common. Cars have the same issues, but they have seatbelts that keep you from flying through the windshield or slamming into the steering column. As for whether it's dangerous - in general I agree with you. I've been commuting by bike 2000 miles a year for 10 years, and have had only a couple of incidents, but when things happen, I'm glad I have a helmet. Similarly, I wear a seatbelt when driving, even though I've never had an accident that necessitated one.

While I agree with you, the answer is that you are more likely to come in contact with moving cars when you're on a bike. It's mostly dangerous because drivers are not used to dealing with cyclists (assuming the cyclist follows traffic rules of course). In my country (Holland) we have completely separate bike lines in lots of places and I suppose that also makes a big difference.

What do you do more often? Fall exactly on the top of your head or on your side? In the latter case, the helmet makes your head bigger and can cause your head to rotate. And swift rotation is worse for your brains than a head-on hit.

So only use a helmet when roadracing or mountainbiking, not for a simple city ride.

You seem to be assuming that the only thing you're going to hit with your head is the ground.