DM's are not suppose to add loot like that, and I believe you are under estimating the actual number of spellbooks we've given out. We do understand its an issue however and it is something the admins are looking at.
Off the too of my head, There was a spellbook in early season 1(flameskull?), one from Azbara, and one from his boss in S1. There was boltsmelters book and one from Aerisi in S2. 2 from royal oozes in S3, and one so-far in S4. So yeah, I underestimated. There are many expeditions I've never gone through so there are probably a couple I dont know about.
Spellbooks are mundane gear...DMs are explicitly instructed to add in mundane treasure in many many places. Especially in places where AL deliberately deletes magic item rewards that the designers intended the PCs to find. If they want to fix that all they need to do is get WOTC to eratta the value of spellbooks with spells . Since they won't even errata obvious things like Simulacrum , contagion and Phantasmal Force, I don't really see that happening.
I have noticed the effort to have at least a 'couple' spell books exist in the world. I very nearly decided to go on a Fabricate crusade with my wizard, disseminating copies of spellbooks across the land to spread folks interest in magic. If I could go to cons I would have been passing out my own custom little spellbook certs to tables, with a little back story and a makers mark on each one.