Why do 99% of teenagers who skateboard not wear helmets?

A lot of the helmets-- especially the older ones-- look bad. Like bicycle helmets look funny especially when their pink.

Trying to 'show off' and/or to look cooler and better than everyone else.

Natural selection.

It’s a sport and I have to do it

It’s a way to help the gene pool.  

It’s not cool, duh! I know it’s cooler than brain damage but teenagers dont do risk assessments.

Mainly because they are uncomfortable. Its hot and irritating. Ive skated most of my life. Generally I dont wear a helmet in low risk sessions. When I am trying something new or doing something more advanced I put it on. Once I'm confident in my ability to be safe, ill ditch it. But I always wear one if I'm dropping in a pool or on any kind of vert or any kind of advanced dangerous grind or anything of obvious risk. 

Most skaters do flat ground tricks and its uncommon for head injures.

that's fine - it's their life, their responsibility

'Cause they badasses.