Because the ways in which this usually manifests is "hurry up" culture. How is "great work" measured and what is the manager's responsibility for assigning/distributing work that is appropriate to each employee?
I view the manager's role as one that clearly defines and communicates business imperatives and facilitates communication about how to get there. If you're telling people exactly what to do, how does that enable them to do great work? Aren't they just doing what the manager tells them to?
Sometimes, but there are also people who just want to accomplish the task at hand and collect a paycheck. And for the enthusiastic employee, a good manager should be able to tell if someone is over-committing themselves and setting themselves up for failure. A good manager should be able to deal with a range of enthusiasm and adjust accordingly.