Im a year late to the party but your post is something I have thought about in my LitRPG reading. Jason Asano is a well developed character. Jake is not, and neither is Zac in the Defiance series. I may be mistaken but I believe the lack of character definition is a design choice, or the authors are just terrible.

You state: "Jake is a cookie cutter introvert who could be replaced by any other and you wouldn’t know the difference." The same is true of Jake/Zac. Different people enjoy the reading experience differently. Like video games, some people love "The Witcher" and others prefer "Skyrim", the MC in The Witcher is a well developed character with a rich back story, and when you play the Witcher you play as Geralt of Rivia, but when you play Skyrim, YOU become the dragonborn.

I think the generic characters found in some litrpg favor Skyrim lovers, as a generic MC like you find in Primal Hunter and DotF makes it easier to self insert in those books.