Lore is tremendously important to the game. And here's the amount of lore that setting books have for 99% of monsters in the game: None.
It all comes from the core books and the monster books.
Setting books change the default lore for a tiny fraction of creatures, because those are the ones really important to the setting. But you're not going to find that the Bulette works greatly differently between Greyhawk, Dragonlance, Forgotten Realms, Dark Sun or Ravenloft. (You might choose not to include it, but setting books aren't going to explicitly exclude lots of creatures).
And how many games don't use a published setting? (More than 50%?) Those ones default to the core books lore and then get changed as required. But you want that baseline.
Now, you probably don't want 2E levels of detail on creature ecology and the like. Probably. (Some definitely do!) But especially when creatures have certain abilities, there's a lot of implied lore there that occasionally works a lot better when explicitly described.
Cheers!