There's a bit of old wisdom that says that you should do what you love doing, and that you should never be unhappy with your career for long. I work with some people who say that they do do exactly what they'd most like to.
The first thing that comes to mind for me is, sheesh, you really need better hobbies.
I think that there are a few lucky occupations that are, indeed, exactly what that person would most like to do in the entire world. I'm thinking things like high-profile scientists for a fusion research lab... things that you're passionate about and can only do in that setting. But those kinds of jobs seem like a small minority. The other thing that comes to mind is a stay-at-home parent; if that's what you'd most like to do, that's something that you can generally manage (at least on a technical level) with a few nights' worth of effort.
It's hard for me to really imagine why one would want to be a semiproductive member of society if you don't have to be. I mean, I love my job; I love software. But... I could sit at home and write software. It wouldn't be as big, or as complicated, or affect the lives of nearly as many people... but I could do it. In fact, if that's all I did, I bet I could write something pretty great, and the internet would let me share it with the world. Or, if I felt like it, I could take a month off and just play games for an entire month. Maybe I could work at Microsoft, but not have to ever do what my manager wanted me to do... just come in when I felt like it, leave when I felt like it, and do whatever I wanted.
All of these fantasy scenarios seem better than a realistic one... like my current life. The mere existence of them as very unlikely possibilities makes me scoff anytime someone makes the remark that they already do what they would most like to do. Sure, in the realm of realistic jobs, I'm pretty darned pleased. But I can certainly imagine something much more appealing.
So, maybe if those people don't need better hobbies, they at least need more active imaginations.