That's not always the case, though. I have a friend who works in public accounting who actually does work 80-hour weeks during busy season (i.e. just before taxes are due) and spends pretty much that whole time in the office from what I understand.
quick edit: he works with businesses, too, so he does 80 hour weeks in Oct/Nov. and again in Mar/Apr. and then 40 hour weeks the rest of the year.