Today's generation is used to instant gratification.
I want to earn lots of money now.
I want to be an expert now.
An idea of a 15-year career sounds crazy to us -- we have no idea what we will be doing in 15 years, but it will almost certainly not be the thing we're doing now. Because of that we're limiting ourselves and our potential to the kinds of work that reward bursty, energetic types but don't stick to their guns.
In doing so, we are missing a lot. The most impressive careers are not those that are made overnight, but those that have substance, purpose and meaning. Have you seen someone's life work? A work of someone who has been working on the same thing for 15 years? How about forty years? How impressive is it?