Because we're not machines.
And because not everybody works on what he loves.
Sometimes, the things you love are not actual jobs (not rent paying ones, at least). Other times, they are very competitive and only few can land those jobs.
>This is an important questions for programmers and engineers, because we generally enjoy building things.
We also enjoy other things. I like programming at work. I like programming at home. I like reading about programming. I like participating in programming forums (well, it's kind of obvious, since it's Saturday night were I am and I'm on HN).
But I have tons of other stuff I'm interested in.
Not all if related to my work -- or even programming in general. And, yes, from time to time, I like to "veg in front of the TV".