Personally, I think it's because being a teenager is difficult and it's hard to have any set belief in something when you're having trouble. Some people may say that teenage problems are stupid and immature, but think about how you felt when you were a teenager. Think about the things that bothered you. It didn't matter how insignificant or immature they were, all that matter is that they bothered you in that moment. In that moment of your life you didn't know where to turn to.
My opinion on god is that he indeed lacks a presence on earth. People say that if you pray god will help you. However, all I have learned about god points to the opposite. God gave us free will. God gave us the ability to decide and create our own path in life. In order for this to be true he cannot interfere. So there is nothing but faith. But, for a teenager, it's hard to have faith when there is no evidence of something being real. And will all the controversy the different sects of christianity have about their own god, how would one even know what is right and what to believe in?
Personally I do think there is some higher power. There is purpose to events in life, though I'm unsure about a grand purpose or meaning to life.
A lot of it is that everything seems to be in black and white. You believe or you don't. Most people probably believe that you have to be part of a religious organization, and follow all their beliefs, to claim a religion. And being agnostic isn't something you hear of every day. I think mostly it's because people who are agnostic, since they don't tend to lean to any particular side, don't really argue about it. While atheists and theists are at eachothers throats, agnostics are going about their lives.