There's obviously many factors at play here, a lot of people have mentioned different brain chemistry, interest in the work your doing (which of course also isn't a binary choice, you can enjoy your job but enjoy your personal projects more), etc.. But I think one thing that doesn't get talked about enough is, if I were to try to put it into a word, finality.
I can work for 4-6 hours and be completely wiped. Equally, in the past I've done ludum dares for 24, I seem to remember once even getting close to 48 hours straight, and at the end I'm full of beans (or more so than you'd expect). Again, obviously interest plays a part here but what is much bigger in my opinion is that I know that at the end of that 48 hours, my work is done. I'm "free". If I were to do 48 hours straight at my office job, I'd still be back in the next day, and the day after, and so on.
If the work has no end, what is the motivation to power through it?
I can work for 4-6 hours and be completely wiped. Equally, in the past I've done ludum dares for 24, I seem to remember once even getting close to 48 hours straight, and at the end I'm full of beans (or more so than you'd expect). Again, obviously interest plays a part here but what is much bigger in my opinion is that I know that at the end of that 48 hours, my work is done. I'm "free". If I were to do 48 hours straight at my office job, I'd still be back in the next day, and the day after, and so on.
If the work has no end, what is the motivation to power through it?