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.
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.