Agreed. I think this is clearest when you look at benefits, e.g., the many American companies that only offer health insurance benefits to their full-time workers, but you see it in terms of pay, too. Most middle and high-paying jobs are exclusively full-time (and some expect 50, 60, or even more hours a week).

I live in a country where health insurance is individual, not employment based, so maybe that's part of why we offer 80% (most common) but also 60 and 40% positions?