If a researcher finds the counter-intuitive results that working less leads to equivalent or better outcomes, it will be a headline.
If a researcher finds that working less leads to getting less work done, who would publish it? It won't get media headlines, but even scientific journals will not accept the results.
Though I'd argue there's also a bias in that there are likely very few researchers choosing to hypothesize the five day work week is more productive, structuring the study to confirm that, and executing the study.
The incentives are all in the other direction.
What? What are you talking about? Why would a scientific journal not accept it, if it's a properly done study that contradicts recent results?
Furthermore, there are plenty of "water is wet" studies that get published all the time—some of which are confirming things we "know", but don't have solid scientific data on, others of which are doing replications of existing studies. (I suppose you probably wouldn't get much traction with studies that rehash very old, many-times-replicated results...)
I don't think scientific journals work the way you think they do.
Individual firm metrics are useless to even bring up here.
The question is finding the balance, work that produces productive things people value. And it turns out, free market economic systems are pretty good at democratically converging on answer.