probably because most people don't want variation? if you have to work forty hours a week, you might as well work the same forty hours as your partner/friends/family so you can easily schedule non-work tasks and social events in the evenings or on the weekend. if you have children that need to be shuttled to/from school, 9-5 aligns well with that.
ah I see from your website that you are questioning the forty hour workweek to begin with. I'm sure most people would happily work eight hours less for the same pay, but I'm not sure how much interest there is in 80% time for 80% pay. also going back to my previous point, the value of that extra free day is diminished somewhat if it doesn't align with activities you want to participate in or availability of people you want to spend time with. I guess you could just treat it as a "chore day" to free up more time on the actual weekend.