PG is an excellent writer, but this essay seems remarkably misleading. The unstated premise seems to be that well-educated adults already know everything they want/need to know about everything, which is silly. I'm older than PG, and pretty well educated, but I am constantly learning new things. I don't think it's because they are not important or I am obtuse. I think it is because I am (still) intellectually curious.
Sometimes I learn new things because they are new. And sometimes I learn new things (that are well known to people in other fields) because while I know a lot about some things, I know very little about others -- so little that I don't even know those things overlap with my interests.
Those of us who enjoy learning appreciate that we will never know everything we would like to, and in fact we will never know the boundaries of knowledge for topics we care a lot about. It's not that it is unimportant to us, it's just that we hadn't learned about it yet. That's why we read essays.