That is to say: There's an obvious use for a focal point in this industry, so this facet of dath ilan has carefully put together that focal point, since there's not room for multiplicity.
One could perhaps go on a bit about how this edited compendium is assembled, by verdict of fandom and verdict of prediction markets, to contain all the most successful bits of isekai protagonist self-improvement that has done well out of previous books on the subject--along of course with references, and thereby advertisements, to the isekai-protagonist-self-improvement books they were taken from--and what happens in terms of royalties if a later author devises a marginally improved version of a rhyming parable for how to make sulfuric acid, where most of the innovation was in the earlier version--but it's not the main point right now.
The point is, declaring that a protagonist happens to have read the seven volumes of Self-Improvement for Isekai Self-Inserts is practically a free action in terms of the story's burden-of-improbability; tons and tons of people have read those volumes in real life! Your protagonist doesn't even need to be an isekai fiction writer! People will read those books just because they're interesting! And because almost everyone in the world feels deep down like how well they would do in an isekai scenario is in fact a valid measure of their own self-worth!