Why on earth would knowledge not be useable in combat?
Because some of don't play D&D combat as a deliberative, slow-motion meta-game. Each round is 6 seconds. Most combat takes place in darkened caverns, amidst shrieks, yowls, and the clangour of battle. There is not the time or the clarity to coolly assess what exactly you're fighting, cross-reference that with your knowledge of flora and fauna, and then communicate that to your companions in the midst of a wild melee.
Want to use your memory from books and lore to identify the creatures shuffling around a camp that you're spying on? Sure. Want to use your memory of books and lore to identify the tentacled horror that grabbed your shrieking companion from a pit in flickering torchlight 4 seconds ago? Not possible.