Why admirals rarely chose to return to captain

Have admirals recognized that they were poor at the job and voluntarily returned to captain?

No; the explanation centres on the strong self-belief required to make decisions that risk other people's lives.

Command requires enough belief in one's judgement to make decisions that risk other people's lives. That confidence is a form of ego, even without boasting about being the best. The distinction is between self-belief that supports a decision and an inflated sense of personal superiority. Persistent indecision can endanger both ships and their crews.