40 hours really has to due with overtime rates, advocated by progressive companies back in the day. 20 years ago, Progressive companies advocated for company gyms, day-care, unlimited M&Ms and ping-pong tables. Now, its work from home (for now.)

Otherwise can we question the need to differentiate full-time vs part-time? Its certainly arbitrary, and mostly related to whether one qualifies for benefits. In the case of benefits, it may be less than 40 hours.