Because it's an out of court deal. We all know beyond doubt that he at least met her. We've all seen the photo. What his representatives did was say to her "listen, you can go to court, you'll have your say, you might win, against the finest legal minds in the business, you'll have the entire sexual history since Year Dot dragged out and printed around the world, you'll have a miserable time, for months of maybe years, and IF you win you'll get a sum of money. Alternatively, we can give you an astronomical sum of money, a private apology, and in exchange you will never, ever talk about it again in public. Now sign here, and bear in mind that if you ever break this agreement we'll have it all back, and more, with costs. This is no public admission of guilt, and if you so much as breathe otherwise that's breaking the agreement." Etc .
You will have noticed that the woman in question has absolutely disappeared from the media and has been as silent as the grave ever since the signing of the cheque. That's the deal.