that's because being good at your job is necessary to being a good manager. it's not just managing people. if this was true, then you can take any asshole off the street and make them manager. this example is more easily explained with something like an army. the general better know how everything on the battlefield works and how to train and motivate people but he doesnt have to be the best shooter. if you drop a general onto the battle field has a infantry, he'll do just fine too.

sometimes this situation is confounded with the fact that not the best people are being promoted. sometimes execs want someone they can trust to run things and not necessarily someone who is the best at managing. so when you see a bad manager, you think managers don't need to be good at the jobs they're managing but it's not true.