You can think of it sort of like a variant of the Peter Principle (people are promoted to their level of incompetence), but instead of being promoted and given more/different responsibilities, people are just given more of the same work.
The “most anyone can do” requires an unsustainable effort that leaves them exhausted and worn out. But Burnout is only part of it.
They need to feel progress and growth so that their time and effort hasn’t been wasted. A tight deadline or overwhelming workload doesn’t require increased mastery though, it only requires increased endurance. Doing the same work you already do well doesn’t make you better — it makes you bored.
Growing your career isn’t extra hours to ship a few more units of 1work,it’sdevelopingthemasterynecessarytoproduce2 work.
But that doesn’t mean the harder work needs to be stressful or painful. If management is doing a good job, people should have a fairly constant challenge level.