Its the same logic as why PhD programs don't simply double their size when there are plenty of people with enough credentials to pass entrace exams. Its a people-management issue, and maintaining the incentives of the rank-higherarchy. It has nothing to do with talent levels.
These positions are only loosely meritocratic. The higher-order filtering is almost always based on some form of social privledge. Its not dysfunctional, at least in a litereal sense. Privledge opens doors and changes the economics (ie, productivity) of the industry materially. That is why it is selected for very heavily.