Well, to be frank-- you're right. To some extent we are all the arbiters of what constitutes satisfactory learning.
My experience of grad school was that it aimed to modify how I learned.
To a certain extent, newly minted PhDs are still only experts in the barest sense, but getting a PhD is the point at which people become more-or-less "on their own" as scholars... when they publish work, it's no longer "student" work, it is work that is put out by a person who has passed a qualifying exam and defended their ideas against a panel of peers.
This is a process of how new academically-blessed authors are created: it's a process of authorization. There's a certain amount of authority that this conveys to say "I've learned this stuff, here's what I found." That "authority" only exists within the framework of academia, and we are all free folks who have that authority from birth.
So yeah, you're right that we have this authority in general from birth. But to a certain extent, gaining that authority in that context is the process of pursuing a PhD, and once you have that I am not seeing what there is to gain even within that context.
Does that clarify what I am wondering?