Her refusal has to do with what is fair to Charles as well has to herself. If Charles cannot make her a happy wife, then it isn't fair to him to marry him just to establish a different household. As strong as that inducement may be to us, it isn't strong enough for Anne. She is practical in many ways, but she is also romantic. Charles doesn't know that she'd be "settling" for him, and I think it's definitely better for both of them if she refuses to settle.