Because their union insists that all their members get a 4-day work week or you don't get any workers at all. This is what unions are for: standing up for workers' rights when the employer would otherwise be incentivized to pursue the short-term gains from exploitation.

Or the workers can go to an employer without a union, work 5 days a week, and see a greater return on the amount of work that they're putting out since they're having 20% more output than the competition without having to pay union dues

> A stressed-out employee who is at work is going to outperform a well-rested employee who isn't present

Citation needed.

There is no hard evidence for this belief. In fact, and especially for creative jobs, employees that are well rested and happy will contribute more than stressed out plebe barely getting by.

You ask me for a citation, then make an even more specific claim without providing definitions or a citation yourself?