Earlier quoted context omitted.
Why would you pay them the same? Hire 14% more workers, work them 35h instead of 40h, and pay them 14% less each. Costs the same, doesn't it?
Many people will not be happy to get a 14% wage cut, and would prefer working longer. Also, firms have a tendency to just require people to work harder/overtime rather than hiring more. Finally, retirement benefits decrease if they are awarded based on how much income taxes you payed, which not everyone is comfortable with. That is why projects to have a legal 35 hours week usually have extra stuff attached to them,…
In the context of the public sector, one is mostly shuffling money around (welfare vs pay/pension) -- but with tangible health effects: for better or worse, very simply, people are generally better off if they have a job, if the job isn't too hard for them (social aspects of having co-workers, sense of worth etc).
I'm not sure about Sweden, but Norway in practice (through union regulation) have a 37.5 hour work week. And a six-hour day then means 30 hour week. Some sectors do have a 40 hour work week (8 * 5), but also here six hour day, would generally mean 30 hour week.
In Norway hourly compensation is still not the norm (except for overtime) - nominal hourly wage is based on a yearly pay divided by 1950 hours (37.5 * 52 - including holyday pay). I think the idea of paying people per hour worked is flawed in general, even if it seems good on the surface. You pay people to be an employee, to use their skills to help a business or organization succeed. I don't think you can often gauge a persons value by hours worked, and it makes for odd incentives (eg: you get paid more, the slower you work).
Again from the perspective of the public sector, I think, in general, that one wants people to have a "full time" job, and pay taxes accordingly -- which implies they should be paid accordingly. That is part of the premise of a six hour work day -- that you can get "a days work" done in six hours, that working more isn't really good for you or your employer in general. Things like sick leave, efficiency when at work, years of a life that can be spent working -- all points towards six hours making more sense than eight.
We probably do need more data -- we have much more experience with eight hour days/40 hour weeks than with six hour days.
I also think it's important to avid the "one size fits all trap" -- not all work is the same. But that also means that just because Ford thought 8 hours was great for factory workers, it might not be the best model for most modern work.