Well it's a good question! I believe this is due to the structure of our society and the lives we lead within that society. Like you said, all of us had dreams when we were children, but somewhere along the line we abandon them. Why?? Well, by time most of us start our wretched monotonous working lives we get into a particular pattern of living, a precise routine, we don't like it! But we figure "hey I'll just do this for a little while then I'll call it quits on my terms" by then, it's too late, you've got a mortgage, you're thinking of settling down and getting married, and you've just got your first promotion and you reconsider, you say to yourself "after all, I'm living a relatively secure and comfortable life. Maybe this isn't what I always wanted, but hey, who gets what they want in life, no one, right??" (I like to call it the Urban Adams Apple).
Someone once said that Lack of will power has caused more failure than lack of intelligence or ability, and for me that sums up the whole problem perfectly. It's not that most of us lack the particular talent to succeed in our chosen path, it's just that most of us opt to give up after stumbling on the first hurdle. After all, if you look at the history of humanity, of all the billions of souls that have graced humanity it is only a small handful of those who have shaped it. Why? Because most of us like to bemoan our wreteched state of affairs and blame luck, others, whoever, whatever rather that assume responsibility. And until we take responsibility for our actions we're never gonna achieve anything worthwile.
Now I don't want to act as hollier than thou because I will be the first to admit that I've fallen for that trap just as most of us have. I will pursue my dreams one day, but until then I'll just enjoy my sweet reveries