Have you ever smoked a blunt?
Because I don't think you have. If you did you wouldn't say because "young people" think it's cool. First of all, blunt smoking has been around for quite some time. (Mostly black people smoked blunts)
People don't smoke blunts because its cool.
You can put more weed in a blunt. And blunts burn MUCH slower than joints do. Especially if you use certain brands of cigars, mostly cigars with real leaf paper. Optimo and Garcia Vega Senators are perfect for slow burning.
Also most blunts will go out after a while if you don't puff on it. Joints will keep burning.
Also when you're at a party with a TON of people blunts come in more handy than joints.
So, while maybe you think people roll blunts because it's "cool". Most people roll blunts for these reasons. Nobody is telling you to do it, so don't knock people who do it.