Comment by gensym

There's a fixed cost to every employee. Health care being the biggest, so you don't save 20% by dropping an employee to 4 days / week, even with a proportionate pay cut.

Though the bigger reason is the belief that people who are willing to take a paycut in order to work less are not the people you want on the team. There's still a stigma to not making (or least pretending to make) your job the priority and treating every other part of life as a support role for it.