Managers who have the power to do this can't allow their teams to work any less than the "default" 40hr week, because then the first time any outcome doesn't go

Managers who have the power to do this can't allow their teams to work any less than the "default" 40hr week, because then the first time any outcome doesn't go as planned, the person most incentivized to knife them in the back in that company's politics has an easy way to do it: "See, we would have hit goal xxxx if Steve's team wasn't a bunch of lazy SOBs who take Fridays off!" Heck, in many companies I've been in, I've even been made to feel inferior/lazy for not working a bunch of (obviously free) overtime. And I've never even worked for a startup where there was a realistic possibility of stock-option riches, where I can understand why some would be willing to make personal sacrifices for a couple years in the hopes of life changing exits.