Cody Delistraty writes in The Atlantic about why overwork is bad, but we do it anyway:

even though the amount of time you work tends to match how productive you are as if on a sliding scale, length of work and quality of work at a certain point become inversely related. At some point, in other words, the more you work, the less productive you become.

So why do we do it?

Some of the explanation is cultural. University of Pennsylvania professor Alexandra Michel “concluded that hardworking individuals”— in her case, investment bankers— “put in long hours not for ‘rewards, punishments, or obligation.’ Rather, they do so ‘because they cannot conceive otherwise even when it does not make sense to do so.'”

There is also a belief in many countries, the United States especially, that work is an inherently noble pursuit. Many feel existentially lost without the driving structure of work in their life—even if that structure is neither proportionally profitable nor healthy in a physical or psychological sense.