Because there's no rule against doing so.
Also, and weirdly enough, some uploaders claim they get feelings of remorse! They say they want to protect the girl from people on this site by withholding her name.
If only they had a thing called a logical brain, it could point out uploading a girl you don't want to be creeped on is not gonna help said girl
This topic has been beaten to death in other threads:
Idea to tackle nameless uploads
Why do people persist in posting pictures without names? What's the point as they can't be saved anywhere. Surely if you spot a picture of a girl you must know who she is??