1. They only work on what they like.
2. Their work exists for its own sake, not to accomplish some other motive (e.g. the painter's paintings exist to be enjoyed as paintings. It's not graphic design, which exists to sell something else.)
In practice, that's usually not true. A founder deals with compliance forms, a novelist only writes a vampire book because that's what the market wants, and the scientist applied for 27 research grants this year.
When it comes to work, humans seem to need to believe that perfection exists, that there is a situation in which all of our problems would be solved.
I've been lucky enough to have had multiple situations where I reached the exact career goal I set a few years prior. The career that seemed like the situation where all problems would be solved. I was (and continue to be) happy with my career, but as soon as you live in yesteryear's idealized reality, you realize that even within that exist some annoying things and you idealize the next thing.