Perhaps your answer lies in Daisy's first words,
"I'm paralyzed with happiness."
Of course, she is teasing as she is inclined to do, but there is quite a bit of truth in her words. She lives in a beautiful Georgian house in East Egg; she has a daughter, and her husband is very wealthy. She cannot change from her status quo because she has all achieved what must be the height of success: possessing whatever money can buy. Her relationship with Tom is based on his wealth, and it is this wealth that keeps her close to him.
We see that there is tension in their marrriage early on. We know that Tom may be physically abusive toward Daisy, that he is racist, that he is having an affair, which Daisy knows about. Yet, she teases Tom and seems fairly content with things as they are. In fact, even Nick notices after her defiant claim, "Sophisticated--God, I'm sophisticated!" that she is asserting
her membership in a rather distinguishing secret society to which she and Tom belonged.
Nick half expects that Daisy will follow him with her baby in her arms when he leaves, because he senses the tension in their relationship. But he soon realizes that there "were no such intentions in her head." Despite their difficulties, Daisy and Tom are united. She may not like what he does, but she is not going to leave what she has with him. She is indeed paralyzed, not by happiness, but by wealth.
I think that it makes sense that the introduction of the Buchanan marriage is Jordan's disclosure of the infidelity in their relationship. Nick's astonishment and belief that Daisy should exit from this configuration is only matched by why she would stay with him. I think that the only answer that is provided would be the presence of material wealth. The description of the house as well as all the trappings of wealth help to bring out to the reader that Daisy and Tom are very well off. This could be the only possible reason as to why she is with him. The white dress that Daisy wears might be reflective of how impure the marriage actually is in which two people remain together for money and for social status. In the end, this exposition or setting details help to establish the basis of both the relationship as well as a primary motivation in Daisy's personality that will play itself out more as Gatsby becomes more driven to have her in his life.