Training

 Imagine, then, two people of the same aptitude.  

One reads voraciously, learns languages, trains himself to write, learns about a variety of other subjects, (history, art history) plays music, figures out how to succeed in academia, rises to the top of an academic field.

The other: pursues a non-academic career, does fine. Main hobby is watching sports, maybe.  

The variability is in two factors: the opportunity, usually provided by a certain amount of "privilege" growing up, and the drive to do actually do it. I had both. I could have easily had the same privilege and same aptitude, and done nothing with it. 

The training is about 90% of the academic's value. It is work: training oneself to do all these things at a top level, but also expanding the brain through music and language learning. The non-academic could have done it, but didn't.   

Only a reduced percentage of the population get a doctorate or equivalent.  Even for a smart person it is not easy. 

Of these people, only a few get to research jobs and publish. Of these, a certain percentage is more or less on the top. It is an elite proposition, and it takes a lot. This is why academia, structurally, is elitist.