Resources on the Ruy Lopez for White - why so few? The Ruy Lopez is one of the most popular chess openings for both sides after 1.e4, and despite this I feel like there are few good detailed resources for players of the white side wishing to play the main lines in the Ruy.

What makes this seem so strange to me isn't just the relative underrepresentation in comparison to popularity of the opening, but rather that it seems like there are plenty of good resources for white against other major replies to 1.e4 (for instance, GM Negi has personally managed to produce well-received, complete repertoire books against the French, the Caro and the Sicilian (and the somewhat obscure Philidor defense)). Also, I have noticed that there are complete repertoire books in some of the mainlines in the Ruy, but these seem to be from black's point of view as far as I can tell.

My question is therefore: why aren't there more resources on the mainline Ruy from white's point of view? Is it difficult for authors to find good lines for white to recommend, making them hesitant to publish anything (for instance the Berlin giving players on the white side too much of a headache)? Or are there other reasons, like the mainline Ruy being more easy to play from the white side, making authors feel like you're supposed to figure it out by yourself as white?
I personally think the task is too daunting.  There are so many defenses for Black to cover. Some sideline like the Worrall or the exchange Ruy Lopez are each full books on their own.
Secondly, the subtleties are highly abstract and difficult to explain in a book.  I think Karpov could write a book similar to his book on the English opening where he doesn't try to cover variations but instead just annotates a bunch of relevant GM games.