Such an effort is also quite different from salaried work. When you are taking an undergraduate course you put a lot of effort into a finite project. You tend to have the energy reserve for something bounded like that, but it does come at a cost. Usually, at the age where you would enrol in an undergraduate program, the gains outweigh the cost, and the cost is often lowered significantly by the intrinsic motivation of working on your own interests.

With salaried work the only effective limit is your retirement. Working more simply means more income, but if that extra income isn't needed, or doesn't net you enough extra benefits compared to simply having time off, the cost of being mentally engaged for such a large part of the week just doesn't weigh up to the gains.

Sure, if your work is so engaging and rewarding that work itself is a pleasure, than it might pay off. But for most of us it just doesn't work that way. Work often means doing things were mostly others set the agenda, and while you may be good at what you do and find motivation in doing it, I've found that it rarely means that you can do it with the same sustained level of energy and quality for more than four eight hour days — and even that isn't a given.