The free market could do that without unions. Doing so increases the cost of labor in the product as % of the total price. You're super highly valued employee, your employer will be more than happy to buy your work in packages of 4 days instead of 5 if it suits him and you. Also if this is not suitable for one party of the deal (either employee or employer) both can go and freely trade/buy their labour.

However, generally advocates propose a blanket "mandatory 35 hours week", which have many negtive consequences:

- Why do you need to "enforce" that to other people who can't or wan't earn the same way and are more than happy to work overtime because they need to say earn more to pay medical bills or want to save to buy a house? Isn't that limiting the amount I as a person can sell my own work hours to the business?

- How can the business compete on the local market when other companies aren't forced to do work with the same cost base for the labour component in the final product?

- How can the business compete with the Mexican company across the border who can do it for even cheaper?

Free markets are very brutal and at the first glance are bad for humans, but their efficiency gives the tax base for redistribution. Also they're inherently moral, because if you can do something for your fellow citizens and swap your labor for their money and back, then you shouldn't expect to be entitled to their surplus earning redistributed via the welfare system.

In tribes in the olden days, when a person got sick/too old, many tribes just left him to die, because they couldn't afford to feed him. Societies are much wealthier now, but we shouldn't forget that starvation and poverty are the default state, not the other way around.