Not all companies want their managers to be great leaders—it depends on the leadership of the company.
Truthfully, many companies would be more agreeable to producing better followers than better leaders for a myriad of reasons. It could be the small-mindedness of the people in the top-jobs who feel threatened when a subordinate outshines them — and, realistically, there is a startling lack of rewards for mentors involved with mentoring relationships.
There is the traditional (still vaunted) top-down/command and control model of leadership that all but insures rewards for being good “soldiers” vs. great thinkers/do’ers. When an organization treats leadership as something that is exclusive and reserved for the elevated few, then it is forfeiting the engagement of the entirety (body – mind – spirit) of its members.
It seems that no matter how many times this is proven to be a competitive disadvantage, it is still vainly pursued as if it were a birthright. I shant be the one who would state that everyone has an interest in being a leader, nor will all volunteers become successful leaders, but to discourage interested parties within your walls, due to an out of date leadership model, lack of internal awareness/support or even beliefs in false methodology is stunningly shortsighted.