Permissions, probably. Copyrights and statutes. Knowing the librarians, unfortunately the prestige of their job is more vested in denying you access than giving you access.

I mean it's their job to give people access to information, and they certainly do, but the mark of a professional, in their eyes, is guarding information. It's much more embarrassing for them professionally to give too much access than too little.

LLM training gives them a "respectable" way of bypassing that and give the world their information (which, in fairness, they probably all really want to do if they could).