Not everyone feels safe doing that. Plus you need a plastic hat in Australia.
You always do. The damage you can get from a simple balance loss, or worse, when falling on a car or a curb is scary. Wear your plastic hats to protect your head people!
all this being said, summer time + helmet = lots of sweat.
I know of two separate people who were biking as a part of their daily commute without a helmet, and woke up in the hospital with zero recollection of what happened, but multiple issues (such as one of them being unable to concentrate or remember things).
I still don't know how you're supposed to fall head first onto a curb unless you tie your arms behind your back.
That's because safety is cool, although some people do prefer the brain damage option.