Why do folks not wear helmets?

Saw some crazy-action this morning on the commute. some knucklehead was riding an old-school 21" wheeled, banana seat low-rider, with wide handlebars set up above his shoulders. he really let it rip coming down the longfellow bridge into boston - no helmet, iPod blasting in his ears, then BAM! i don't know if he nicked his wide handlebars against a car or when swearving between cars his narrow 21" tire went perpendicular, but he was airborne - up and over the bars, back down on his head, then rolling - all tied-up in the bike... the traffic light had just turned green, so cars weren't moving very quickly at this point or he would surely have been run-over to boot.

I'm first on the scene, having been right behind him at the time. I roll around to his left, blocking traffic so no one drives over him and ask if he's ok, "yeah... i think."

I stop, put my bike against the railing of the bridge, grab his and he begins moving towards the curb. traffic flows along, no one wants to stop - he's only one of those annoying cyclists. two pedestrians are now on-scene, talking with him - he mentions "nicking" his head - more like slamming it against the pavement. now that he's not alone, bike and body up off the roadway, i think i can roll. i look him in the eye and say, "time to get a helmet, brutha', right?" he nods in agreement - i hope for his sake he does.

3mins later, as i ride down berkley street between some of the Hub's only skyscrapers, a bike messenger rolls a few dozen yards away - ripped cut-off pants complete with thick chain securing his wallet - no helmet of course. no self-respecting currier would wear a helmet... everyone wants to be cool. i just want to avoid being a life-long vegetable!

"hey jeff, want me to put those pureed carrots in the feeding tube? blink once for yes, twice for no." blink-blink.