This is no surprise really. Any major company promotes managers from within unless they have a lot of industry experience. A bank manager would have a hard time being a manager at any FANNG company (on the engineering side at least) no matter how good they are with people...you have to have at least some in depth understanding of what the company and department does.

The best engineers don't usually make the best managers either because they have a hard time giving up control. Good managers let their staff do their jobs and give them the tools needed to excel (including mentoring and coaching).