Part of it is the language. Technical interviewers love hearing about how I wrote a cool program in Lisp, for example, but then never seem to want to hire me to actually write Lisp code. (I didn't learn Lisp just to show off.)
Part of it is the project. Virtually no software project I see these days looks interesting enough that I'd want to work on it. (Most software I don't even want to use, and even if it's free.)
There may well be other parts. But thinking back, I had one job where I picked the language and the project, and did have quite a bit of fun there, even though it was by far my lowest salary.
I would love to be paid for programs I want to write anyway, the way I want to write them. Still working out how to do that. :-)