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Two reasons, amongst many:
Having uploaded a number myself and in other cases tried to reverse image search a number of images, some models don't have a name in the same place as the photo.
For example, we see uploads from Instagram and Tumblr, and they often come from profiles that aren't tied to a specific model. Often the name of the model is missing, so the path of information loss continues.
Another example, is that the model was part of a promotional campaign for a product, but her name was never disclosed, since it didn't need to be.
Staff don't know many of the names, so depend on the community's collective knowledge to add the name when known.