Have you ever been someone's “best boss I've ever had”?
If not why not?
If so, was it because you gave them everything they wanted and let them slack off. Or was it because you achieved something together, because you pushed them to grow, because you cared?
Why is this not a widely-used metric in corporations? Because it is hard? Because the top might not like the answer? Because no one cares?
If your organisation really wants to be famous for people development then why not start measuring it?