Maybe most people can get all their work done in 4 hours so sitting in an office for 8 is pointless
However, management bought 8 hours of your time and not 4 when they hired you.
So the times you’re off on the weekends doing no work is valuable to the company because you’re recharging your mind and possibly also thinking about problems subconsciously.
I went to check my job offer, and it doesn’t mention anything about 40 hour weeks or 8 hour days.
Nowhere in my employment contract does it say that.
Realistically, I am allowed to spend up to 8 hours to work on goals that my employer says I should work on. It is also understood that if the goals are completed earlier, I am free to fill the "free" part of 8 hours with self-learning.
There's also an implicit understanding that certain life activities may happen/overlap with that free time, e.g. eating/picking up kids.
Only in the most pedantic bad take world would your statement be correct.