This is the cycle I keep seeing. It has happened enough times that it no longer feels random. Someone average moves into a leadership role. Maybe they interviewed well. Maybe they know the right people. Then they are put in charge of a strong team. Within six months, the best people are gone. They quit. They transfer. They find something better.
And yet, the mediocre manager stays. The work gets worse. The team gets weaker. Then the company acts surprised. Promote the wrong people and you lose the right ones. It is not complicated. It is cause and effect. But they never seem to learn.