You are right. Some people can get 40 hours done in 30. And others take 50 to do the same thing. If you want to get ahead you know everything about a subject before anyone else. And you become important to someone. Doing a good job, means you will be let go as soon as the job is not needed anymore.
What I discovered as a manager was that if the company mandated overtime, the same amount of work got done but cost the amount of the overtime. Knowing they’re going to be there the workers just slowed down. There is no gain from working overtime.
Also, when I discovered workers spending absurd hours I’d investigate and, in every case I can remember, the worker had a problem at home. I told them, I don’t want you here after 40 hours. If there’s really that much work, then it’s my job to increase the staff.