dave2008
Legend
Need is to strong of a word, it is a preference. I can use a setting specific book, but I find they tend to have a lot of stuff I ignore or need to rewrite / rethink / adjust. The less of that the better IMO. Here are few other thoughts:From your view, why do monsters need to be "generic"?
More monsters: With less setting specific fluff (and therefore less fluff) we can get more monsters. That is a good thing IMO.
More flexibility: Similar to my initial comment, more generic is more flexible without having to put in effort to revise setting info I don't want
More flavor: If the core book is generic, the setting books can be more flavorful. If you are familiar with 4e, it is the difference between the Monster Vault and Monster Vault: Threats to the Nentir Vale.
Or take alignment. The core book should be flexible, Orcs can be any alignment, but usually chaotic evil (or something similar). However, in a setting book you can straight up say Orcs are neutral evil or whatever.