The external authority is reality. There are objective moral and spiritual laws (e.g. "what goes around, comes around") just as there are objective physical laws. Those laws are enshrined in every great tradition, but encrusted with a lot of superstitious and political nonsense that is given the same sacred status as the living truth it partially obscures. Right now, we're offered a stark choice between buying it all or rejecting it all. This is immensely frustrating to many, many people who want to investigate, discover, and submit to the truth, but for whom merely to submit to authority is an abdication of responsibility.