Why competent captains could become poor admirals

Why would a navy make a complete idiot an admiral?

Politics could promote an adequate captain, but commanding a fleet required a different ability to delegate and direct subordinates.

Political connections could promote an efficient captain to admiral without establishing whether he could command a fleet. A captain can walk around his ship, lead by example, and motivate individuals directly. An admiral cannot do that aboard every vessel and must let subordinate captains get on with their duties. Success in one role therefore does not establish competence in the other.