Aviation widely relies on the transfer of institutional knowledge. The flight instructor teaching you to fly didnt acquire all his/her skills alone. Someone taught them the basics, who in turn was taught by another individual, and so on. Lessons from past experience (a.k.a. mistakes) enlighten future generations. ATC is no different. Regardless of background, when a controller walks in the door of an ATC facility, they need to learn how to work that particular facilitys airspace. Achieving certification depends on the experience and guidance of the other controllers and staffers.