Because creating further artificial labor scarcity via work week restrictions will fix a labor scarcity problem.
Labor restrictions restrict productivity, raising prices and reducing what people buy, thus reducing employment. In short: you have less to barter with, therefor there is less you can barter for, therefor somebody who produces something will find nobody can pay them for the product, and so he becomes unemployed.
Imagine you spend 10% of your income on food, 4% on clothing, 2% on personal care, 30% on housing, 18% on transportation, and 36% on entertainment and other non-essential spending. Call it by dollars: $100, $40, $20, $300, $180, $360. You have a total of $1,000 to spend.
Now imagine everything just got 20% more expensive because everyone working 5 days making $1,000 is now working 4 days making $1,000 and, for every 5 such people, we hire another worker making $1,000 to fill in the gap (i.e. that last day costs an extra $200 per person now). I suppose you got this far and then determined there's that extra worker now, right? Let's look at it further.
So now nobody's getting paid more; they're working less, and MORE PEOPLE ARE BEING PAID to make the products you buy. Your expenses are $120, $48, $24, $360, $216, and $432. That's $1,200--or $200 more than you were able to spend before, and even more than you're able to spend now.
Well let's tie it all together. Food, clothing, personal care, housing, transportation... that's $768 right there. You have about $232 to spend on the other stuff you were buying--about 64% as much. 36% of the production related to those jobs is now unsustainable (there's no revenue to pay all those wages), and so those jobs vanish.
That's the point. You create a situation where people have more money to spend than there are workers to supply, and then you boost the labor expense of anything they want to buy by restricting labor hours. Suddenly everything becomes more expensive, but nobody has any more money; the capacity to buy products beyond what our labor force can supply goes away, because we're suddenly all poorer.