The royal family and their advisers tends to view things from a big picture perspective, they like to keep all options open. Until William and Harry start having children, the York princesses are the next senior line of heirs in that generation. Depending on how things play out, and how long the elder generations remain living and active, the Yorks very well might be needed to shoulder some royal duties until the next generation is old enough.
Once William and Harry's children start arriving, the "need" of the York girls as back-ups will start to wane. Plus they will soon end their education. It seems to me that the royals are trying to slim down their numbers, so I'm not sure the Queen will ask them to take on royal duties. That will be interesting to see what is decided. If HM doesn't ask them to do royal duties, they will have to begin their own careers / start private lives.
Finally, the household might be waiting to see if (as adults) the girls turn out to be more like the rest of the royal family - or more like their mother - before deciding if they will be suitable to represent the Crown. Those decisions might be ones HM is happy to leave to Charles to sort out.