Screenshots are not checked in depth unless someone is having productivity issues. Screenshots are actually a protection for the employee who is not getting work done as they should. Sometimes it turns out that they've been working very hard at a specific problem, but may just not have been able to make progress against it at this point.
I also come at it from a different point of view. I don't have a specific number of widgets per day a person is supposed to turn out. That is not what I am paying for. I am paying for that person's expertise and time and it is up to me to direct it in a way that is profitable for the company. If I direct the person to do a test that is not profitable for the company and they do it to their best of their ability, that is my fault, not theirs. I never ask for overtime, but I do require that people work their allocated hours.
I have 200 full-time employees doing a very wide range of tasks and judging productivity is challenging even though we have many KPIs for measuring it. Time is far easier to manage. I do give incentive pay based on productivity to try to encourage not just butt in the chair time, but productive work.
Back when I was a line employee, I wish that there was a way to pay purely based on productivity. Productivity. As a manager, I have seen it is incredibly challenging to devise a proper system to do so. I have spent years and years working on this and still only have systems that are okay at it. Time just makes sense to measure from an employee standpoint and from a business standpoint, that's why most businesses do it.