nobody even agrees on exactly how big it was or how much it actually held, most of what we "know" comes from secondhand ancient accounts, not hard records.
contrary to the popular "one big dramatic fire" story, it actually declined slowly over centuries, partial destructions (including during one of Caesar's wars), funding drying up, general neglect. death by a thousand cuts basically, not one big blaze.
it's become this cultural symbol for "irreplaceable lost knowledge" way more than a precisely documented historical event.