I think the simplest answer that probably accounts for most of it is just that employees all have overhead, 20 people working full time is more efficient than 40 people working half time. Less HR work, less moving context between people, less office space, less laptops, etc.
So I can see how naturally the system has prioritised this way of working, but despite the efficiency I think we need to do something to normalise 20-30 hours as full time. Give people less iphones and more hours in their life.