That's because if a programmer is bad, work doesn't get done, and people notice immediately. But if a manager is bad, the subordinates often manage to get work

That's because if a programmer is bad, work doesn't get done, and people notice immediately. But if a manager is bad, the subordinates often manage to get work done despite the manager, and the bad manager is in turn more likely to have a bad manager who doesn't recognize the on-going failure.