Spellbooks all start off in the possession of wizards, but then, the same is true of scrolls. Sometimes, a wizard decides to make some money and sells a scroll, or gets defeated and their scrolls taken as loot, but the same is true of spellbooks. Plus, it's possible for an academy or other wizards' organization to have a library of spellbooks available (for a fee, presumably), since they're not consumed, but it's not possible to do the same with scrolls, so if anything spellbooks should be easier to come by.