Why do most managers add no value to their teams?

I have been observing and making my bit to help Managers add value throughout my experience of teaching/ training and mentoring managers. My observation on the reasons for why MOST Managers do not add value are

1. They career growth is faster than their Learning Curve (While this problem exist today across the business, but this is more true in modern business segment than conventional business segment).

2. The gaps in demand - supply (in terms of headcount) has also contributed to this.

3. The promotions and hiring are sometime more for managing "headcounts" rather than "heads"

4. Managers often land up getting deeper into task. In a lighter sense they never get into Managing their Resources.

5. Reduced levels of leadership skills among the Managers. I personally believe that the Managers can add value only when they also demonstrate some leadership skill.

6. The term Value Addition is often not understood well

7. Most managers lack in ability to present and articulate

I see this as a problem from Individual as well as Organisation. Individuals need to take initiative and organizations would need to create/ evolve/ define/ improve the Managerial Environment.