The answer was simple when I really got down to it: I’m a fiction writer.
Where specializing can be extremely beneficial to a non-fiction writer because they need to be an expert on their subject, a fiction writer needs to have a grasp, at least tangentially, of a huge range of subjects because we must portray a wide range of humanity in our fiction. Even if we write from experience, say we write stories set on the coast of Maine because we grew up there, we will still have characters with experiences and skills that we have never known. To tell a convincing story, we must be able to paint enough of those experiences and skills in the characters to give the impression of reality, and to do that well, we need to have access to and be able to generate at least some interest in that wide range of material.
It may be years before I delve into my book on basketry, but I have a character in a book I intend to write in the hopefully not too distant future who is introduced while picking reeds for basket weaving. When I reach that point, this book will become invaluable as a starting point for giving verisimilitude to this character. Yes, I could go to the library, but sadly, I found that, while the library occasionally has fantastic books, it also doesn’t always carry what I need.
So we authors, to create and offer great stories, need a thirst and appreciation for a wide range of human (alien, animal, celestial, or whatever we’re writing) experiences and subject matter. I suppose, that’s one advantage to being bibliophiles: we’re more likely to have what we need close at hand.