This, for some reason makes too much sense. So much so that I’m curious if there is some sort of world wide conspiracy to not do this.
If more people were employed during less time, we would also need more administration.
Not to mention that there are at least two sectors where there can never be enough employees: medicine and education.
Rate of profit = Exploitation rate ÷ (1 + Organic composition of capital)
Don't forget about elderly care, which is not a subset of medicine.
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.