Because usually she's in a cage that is inescapable by the prisoner (herself) and/or she is surrounded by Bowser's guards and there's so many of them that it would be impossible for her to escape without one of them grabbing and re-capturing her. But probably also mainly for the reason Princess Viola mentioned. I might also add that Bowser's fortresses and castles are always completely locked down with the maximum level of security so that also would make it difficult for her to escape especially without for example the key to the front or back door which Bowser and/or his minions probably have.
To be objective, I think everyone on this thread including me is right LOL. We all have good points.
Seriously though all joking aside I do think that all of our reasoning behind why she doesn't fight back makes sense.
I think it's ALL of these things.