I partially agree with you, but personal productivity varies. So in our company we have a rule "work as much as you like, usually we expect no less than 30 hours, but it is OK to do 50. Anything beyond these ranges is odd".
Well many managers and execs might respond with "well then, give 'em more work! We obviously aren't giving them enough to do!"
Salary exempt is usually a losing game for developers. Crappy places will say "we pay you a salary to get the work done" and under staff and/or overwork. If it takes you 60 hours then you have to work 60 hours. But if it only took someone 30 hours they can't leave early because a) deadlines will be tightened to ensure 40+ hours or b) more work and assignments will be given.
Same experience. I've used the extra time to start internal side projects, for which a few I've earned $$ for. Surprisingly no one asks how I have the extra time
I used to be someone who worked only 4 hours max in any company and use remaining time to be on top of tech stacks(SysAdmin). But last four years i self trained on stock trading and it close to be becoming my main job in another year or so. Though i now look for trader jobs where i can trade for profit % but the salaries are no where near in my country for traders.
Everyone is different, so good managers set targets based on past performance. If you set the bar at what you can accomplish by working 3-5 hours per day, they'll assume it takes you 8 hours per day, and give you work based on that. If that works for you, great.
Some thinking-intensive work can bring you at the edge in just 3 hours a day. After that, you generate more problems than solutions. Especially mathematicians are prone to this. Similar cases in software development need to be respected.
3 hours of thinking-intensive work is fine. Then take lunch/walk/shower, and sit down to write documentation, update bug reports, chat with a coworker about your ideas, review someone else's code.....
This has been consistent for every job I've ever had.