Why are you so upset about other people's life decisions that have nothing to do with you? You're not retarded, are you?
To answer your question, it is because it feels tight and looks cool. Simple as.
It's like diving in the ocean butt naked; a feeling of nakedness, liberation and vulnerability. Your mind instantly goes into action mode because you know you have to be on the ball or lol u die. Also forces you to not be lazy and brake using your legs no matter how tired you are.
After I rode brakeless for 4 months I decided it was time to get a front brake after one too many close encounters and after realizing that it was retarded to ride without brakes on public roads.
It's still cool though, fuck you.