Well one important financial reason is that the more hours you work, the less money you make per hour, unless you have a huge equity stake. So your effective wage actually decreases if you spend more time at work.
Other reasons are work/life balance, enjoying time to relax, spending time with friends or family, mental health reasons, and things like that.
By that logic you should try to get away with 30 or 35 hours a week (behind your manager's back, if necessary) in order to increase your "effective wage," which actually doesn't get you anything.
The vast majority of salaried employees see no change in quality of life if they work 35, 40, or 45 hours one week to the next. In fact, if you work a lot, one would think you're less likely to go out and spend your money in your limited free time.
The times I've worked 60+ hours a week have always been better for my bank account than the times I've worked 35-40.
Other reasons are work/life balance, enjoying time to relax, spending time with friends or family, mental health reasons, and things like that.