Canada

You have salary workers like process engineers for example. Working less than 5 days regularly just isn't acceptable. I would not be effective in my job if I only had 4 days a week to get everything done. Also someone always has weekend coverage on "off" days for salary or holidays. So you're still "working"

Also many plants have minimum hour requirements in their union contracts where we have to run X days minimum a week or we still pay. There is more to the puzzle than just the office sector.

You have shifts and shift workers yes but again, the mill basically needs to run 24/7. so lots of people get forced for OT, or willingly take it

Edit: I love the downvotes with no refutement. I am not talking from no experience here. I actively work as a process engineer in a steel mill and actively deal with these problems DAILY. Moving to a 4 day week changes nothing in 24/7 operations. You have to run all of the time, end of sentence (or your mill is getting shut down, and you all lose your jobs...nobody in the mill wins there). The compensation is through the roof and most people end up pulling in tons of overtime. I dont know many other jobs that an 18 year old can pull in an EASY 6 figures with no form of education past high school. The hourly guys make WAY more than any of us salary folk (me and other engineers have spoken candidly with guys on the floor, and they pull in well over 100,000 with no overtime), and on top of that, there are guys who get legitimately pissed when they can't get enough overtime or work more hours cuz they want that money