Because society right now is geared towards teaching young people that they're totally awesome and everything they think matters. Our culture right now glorifies people who are young, rich, dumb as shit and obnoxious. And I'm a twenty year old saying this, so I'm hardly out of the target age range.
Who are our stars right now? Teenagers, or very young adults. Today's pop stars and rappers get huge at a high school age by acting like tools and never having to do any work. These are the people we're taught to idolise in society, and an awful lot of tweens and younger kids (not all, but a lot) are too stupid or naive to realise we all hate these people, and that they won't get anywhere in life like that.
Who gets huge portions of entertainment marketed to them? Tweens and teenagers. They get cash from their parents like water, and since they get it without having to do any work, they spend it like wildfire. They have cell phones at that age (God knows I didn't) and vote on American Idol and throw money at material things like it's water.
Suddenly, marketing executives care about catering to a huge youth market, so now every tween believes that the world exists only to serve their needs and their identity. They have a huge sense of self-entitlement, and that they don't need to earn anything because society glorifies everything about young tweeny-bobber douchebags who buy into this ridiculous culture. It's fucking shameful and disgusting.
/rant