If I am still productive at 60 hours and I am being paid a salary, then why not work me 60 hours? Employers care about getting things done, employees care about

If I am still productive at 60 hours and I am being paid a salary, then why not work me 60 hours? Employers care about getting things done, employees care about how their time is spent. Employers need (~)happy employees. I think hours is the right measurement here. Perhaps the issue is exempt employees? I don't see why I shouldn't be paid hourly.