But her hair is a ponytail, hair that reaches the top of the shoulders in these games are usually as far as Maxis is willing to go. Because any custom hair that is long, curly or straight, that goes past the shoulders in all these games bleed into the shirt collars, the actual shoulders, and their clothes. It is not noticable sometimes until the Sim moves. I'm not sure they can actually ever build a longer mesh that falls about the shoulders and longer without it clipping/bleeding into their body and clothes. And those type hairstyles won't work for all tops and clothes.
ETA: It might work if it was not around the shouldes as you described but what if there was an outfit that required a backpack? It would clip into it, so longer hair is usually a no from Maxis because of very bad clipping issues when they move.