Because 40 is sustainable, and the standard we've mostly all agreed to (with some fuzzy room in there for heavy versus light weeks/etc).
This sounds like a reductive answer, because it is: Capitalism is social, zero sum, and reductive. There are people out there who will put in 50, 60, 70 hour weeks. If you do 40, they'll probably beat you, plain and simple. Fortunately, we've all mostly agreed that 40 is enough, so you aren't regularly competing with the people who are willing to destroy themselves for shareholder value. There's nothing all that magical about the number 40. There are numbers greater than 40 that approach "ok, humans can't physically sustain that"; but fortunately we've pushed the number to 40 through years of positive labor movements.
The problem is that typically the burden of work is much more than 40 hours, even if you only work 40 hours. Because those hours are calculated in the most ungenerous way possible.
Your lunch doesn't count. So that's now 9 hours at work. Your morning commute doesn't count. So that's now 10. And your evening commute doesn't count. Now up to 11. And those after-work meetings don't count, but you kind of need to attend them because of the implications. And so on.
While you may "work" 40 hours a week, you certainly contribute to work much more than 40 hours. For me, it's over 60, and from what I've seen that's fairly typical.
This sounds like a reductive answer, because it is: Capitalism is social, zero sum, and reductive. There are people out there who will put in 50, 60, 70 hour weeks. If you do 40, they'll probably beat you, plain and simple. Fortunately, we've all mostly agreed that 40 is enough, so you aren't regularly competing with the people who are willing to destroy themselves for shareholder value. There's nothing all that magical about the number 40. There are numbers greater than 40 that approach "ok, humans can't physically sustain that"; but fortunately we've pushed the number to 40 through years of positive labor movements.