Perhaps people are more interested in looking at the images that others post than they are in finding a model, hiring her, doing a shoot, processing and then posting their own work.
Or perhaps the majority of people are just too shy to ask a model to pose nude. Or maybe they have an insecure and/or jealous significant other and they do not want to cause problems at home.
In the end, there are a lot of reasons why not many people post nude images...
Maybe it is because so many of the younger folks are doing it themselves with their phones (sexting)! Why mess with a photographer when you can do it yourself and maybe get a date to boot.