Well, that's the other side of it. If a company is paying you for 40hrs a week, they want to make sure they're getting their moneys worth.

Of course you can argue about how employees, despite being "in the office" are only productive for 2 or maybe 3 hours of their 8 hour shifts. But that's now how management sees it. Having a person visibly in a chair makes management feel like they are getting their money's worth.

I also don't get how people expect to get paid a full living salary for working less.

Ultimately, at the market level pay rates are tied to productive value. We have a theoretical assumption of 40h/week, but almost everyone understands that it's fictional (and more people are learning that). Most people who are going home and getting the same work done in 2-3 hours aren't working less -- they're working more efficiently, or doing fewer other things (e.g. chatting at the water cooler).

For many types of thought work, 3-4 hours is pushing it anyway; the default assumption of 8 hours doesn't make sense with the heterogeneity of what different types of work actually entail.