[ozay]

They don't wear helmets because they aren't zooming around like Lance Armstrong. The risk of accidents with head injuries a helmet would help against is limited. Same thing why you don't wear a helmet when you go by car although an occupants risk of head injuries in a typical accident is comparable.

[itronitron]

You can't really equate safety gear across different modes of transportation, otherwise if you wear a seat belt when riding in a car then why not wear a seat belt when riding a bicycle?