Because cloning gives leaders a false sense of security. When you have people thinking like you, doing like you, speaking like you, referring to you, quoting you, dressing like you, and grooming like you, then you feel that you're being validated as a leader. You feel that you have value, because other people value being like you. But they're telling you what you want to hear, not what you need to know. So, you may get some artificial harmony, conformity, or uniformity, but you won't have much creativity, synergy, unity, or security.
Cloning comes from insecurity and from being centered on public opinion.
IBM, among many other North American companies, had to learn this lesson the hard way during the past decade. Once the king of cloning, IBM now is a champion of diversity. Leaders and other employees at IBM are learning to value differences. Economic necessity is teaching that same principle to many companies. In fact, I see that diversity and synergy are being championed by most major organizations today.