Is it? 40hrs is quite low by historical standards. 100 hours per week was the norm in the pre-industrial era, and 60+ hours per week was still typical during the Industrial Revolution.

Labour advocacy groups were promoting 40hrs, much like the four day workweek is today, for a long time, but 40hrs didn't actually became the norm until the Great Depression, where capping hours was a tool used to try and spread the work out amongst more workers to try and resolve the high unemployment problem.

> But if you go down to the lower rungs, people are more inclined to compare themselves with peers and tradeoff double hours for the next rung

While that certainly happens, it seems most people in the lower rungs are quite content to work 40 hours per week, even though working more would put them in a much better position. I dare say you even alluded to that when you chose 40 hours in your example.

It is not like 40hrs is the perfect tradeoff or something. As mentioned before, labour advocacy groups have already decided that 32hrs is even better. I expect many people end up working 40 hours just because "that's what you do" and never give it another thought.

> the reason you have dual income households is that they are richer than single income ones.

If we assume both participants work 40 hours per week then it is true that the same household would have less income if one party stopped accruing an income and all else remained equal. But that doesn't necessarily hold true once you start playing with other variables. A higher income party, for example, may enable the household to have a higher income if they work 60 hours per week while the other party takes care of other life responsibilities to enable those longer hours.

A dual income household isn't necessarily the most fruitful option. In fact, marriage — which, while declining, is still the case in most non-single households — assumes that a single income is the ideal option. It seems that "that's what you do" without any further thought is still the primary driving force.