She is lvl 7 when you meet her which is the same in BG2. As far as I'm concerned, the PC from the previous games never bothered opening her cage because she is an incredibly useless, shallow, and annoying character in a game full of memorable, well written characters (such as Edwina). She must have gotten out of her cage eventually but since the only exit out of the BG2 tutorial dungeon collapsed, she spent most of the last 100 years stuck underground eating rats and drinking her own pee to survive. One day some harpers were out meddling in the affairs of others as always and decided to randomly dig holes around Waukeen's Promenade. And that's how Jaheira was found and rescued just in time for BG3.
