I think it's more a matter of having no idea what to look for when you go to buy a bike from a store, which is usually Walmart etc. I remember when I was about to leave for college, my parents and I went shopping for a bike and picked up a mountain bike as it was the only thing they made small enough. For youngsters, there are typically only 2 options, BMX or MTB and neither have fenders. I really doubt it's about them thinking fenders are dorky. But in any case, they usually have fun on any bike, kids don't start getting concerned about mud puddles until they're adults.