It does implicitly answer the "why". If you have more choice in what you work on, you can choose work that you enjoy. If you enjoy your work, you'll do more of

It does implicitly answer the "why". If you have more choice in what you work on, you can choose work that you enjoy. If you enjoy your work, you'll do more of it. It's no longer a paradox when you take into account the fact that high-earning professionals consider work to be "fun".