Some of it is a quirk of the American health care system. Your health care costs the same regardless of your work hours, and it's a significant fraction of compensation. If you were working 80% as much, they would probably only be able to pay 70% of your salary.

(Assuming you were linearly productive, which it's not, but it's the same assumption you were making and is good enough for this illustration.)

There are other fixed overheads, like your manager, human resources, the office itself, and even the cost of hiring you in the first place. It's possible that to make all things equal, they might have to change your salary to as much at 60%.

Of course all things are never equal, so these numbers have really wide error bars. And after all that, you might well decide that 60% salary for a 32 hour week would still be a good deal. I know I'd consider it. But it's important to note that it's going to be rare to find a job that will pay you 80% as much for 80% of the work.