The joke is that they won't do this things rather than can't. She isn't flying or hiking despite her freedom. They complain about kids tying you down while tying themselves down voluntarily without the benefits of kids. Even though she could have kids and/or travel the world, she will do neither. She is an archetype of the modern woman who is distracted by consumerism and greed and laziness and led away from reproduction and family because she has been deluded that the Lazyboy and pizza lifestyle is fulfilling.