1) You’re probably creaming off the best staff, because everyone wants a 4 day week
Which is exactly the point. It’s a perk that people actually want, much more so than fußball tables, or an onsite masseuse. Startups want good people – great pay helps, but often isn’t enough in itself.
People in their 30s are quite likely to have kids. They also have much more experience than the early 20s, and while they make like the kind of work that startups tend to offer, they can often be tempted away by steadier jobs just so they get to go home at 5pm and see their families. Even for those who don’t have kids – good employers encourage a life and interests outside of work, and this gives an extra day for it.
2) You don’t know how much more money you might have if you worked 5 days
Conversely, if you went to a job interview and they said “we actually do a 6 day week here, it makes us more money” what would your reaction be? I’d just laugh and walk out.
When I worked in public sector, I ended up with so many days of leave (and not enough money to go anywhere) that I’d just take Mondays off for months at a time, which was great. I actually asked a previous employer, who’d said they couldn’t afford any payrises for the third year in a row, for a drop to a 4 day week with a slight reduction in salary (so it was a bit more than 4/5 pay, in effect a payrise). They didn’t go for it, sadly.
It’s all down to what is considered “normal”. 12+ hour days, 6 days a week (potentially from the age of 9) used to be “normal”. Working hours have generally declined since the industrial revolution, so no real reason why it couldn’t keep going further.