Because 1-2k years ago, books were very impressive. Most people would only see a few books in their life times (if at all,) and it was for the “rich and smart” people.
Now books are old technology. Pretty funny that some omni god idea main form of communication got badly outdated.
The reality of course is that the various religion ideas fit the various cultures/times they sprung from, but we all already know that, well, we all mostly do, except for the particular god idea that a person believes in gets a free pass.