There are rational cost reasons. The paperwork, training and other overhead costs are mostly fixed per employee. Communication costs trying to coordinate between employees can go up geometrically for complex projects.
There are also a lot of irrational reasons having to do with making everybody work harder and the desire to make it look like everybody is overworked.
Getting back to the original discussion of employment and wages. The idea I have always liked is essentially a universal work program. The government would guarantee employment at some mediocre wage for everybody. It would have it's own set of issue to work out, but it would have advantages in terms of flexibility and reliability. The reliability is the key one, a lot of the reason that employers can abuse employees is that quitting is a huge risk. If there was a fallback job that people knew they could live off, even if poorly, it would be much harder for managers to abuse employees.
ATL cops were told to work and get paid a 4 day week. They now have to go to court on their days off without pay for testimony.
That is actually a different problem and relates to the weakness of the laws that enforce pay and hours worked. A lot of employees are pressed into working overtime, often without pay. The average American now works well over 40 hours a week.
Jay