My suggestion would be not to get into loooooong(ish) discussions about the topic. At the end of the day, you know that unless his dad decides to have more contact no amount of talking to you about the subject is going to increase that contact.
This may seem a bit rough but think about the times when she has asked for a toy you can't buy/give her. You know that the longer you discuss the toy with her, the more convinced she gets that if she insists she will get it.
I know the subject discussed her is not as supperficial as a toy but perhaps a factual response may go further. Something like, ""DD, I agree with you that it would be nicer if you had more contact with Dad, I wish there were, but at the moment, it is simply not possible. Would you like to (insert distraction here, and move the conversation into another topic)
Same goes for being a child of separated parents, certainly that kid has made a HUGE damage with that comment, so I guess it is about reassuring her that being a child of separated parents is not the end of the world, and that despite what many people believe, it is actually better to be a child of separated but happy parents than growing up in a household where people are always angry, shouting to each other and generally unhappy.
When DS was the same age as your DD, he started fantasising with the idea of his dad and I getting together again. What I told DS was, well... basically the truth, that when dad and I were together, neither of us was very happy, that we used to spend long periods in our own, as dad was often away and busy when at home, while after the split he (ds) got to spend more quality time and play more with his dad during contact visits, as before he was always working. I also told him that his dad seemed to be happier and so I was, and although he didn't remember it anymore, he seemed also happier once we stopped shouting at each other.
obviously, I know it is not ideal, I wish I were not a single parent, but well, unfortunately, that's the life we got and we have to make up the best of it.