Why didn't rachel weisz do the mummy tomb of the dragon emperor?

Rachel Weisz was not in "The Mummy: Tomb of the Dragon Emperor" because she did not want to play the role of a mother who has a 21-year old son. She also said that if she had signed on for this movie- then she would have to shoot in China for three months and she was not ready to leave her 2-year-old son, Henry Chance Aronofsky. Other reports suggest that she did not want to be 'typecast'.