The business of signing one's work is a funny thing. Signing your name connotes ownership of your work. That it is good enough to bear the signet of your name. Most artists will usually reserve that final act of signing a work till after everything is finished (right down to the last dot of the pen or the last swipe of the mouse or brush, depending on what medium you're using)
, some will only sign their works if they're satisfied with the outcome, and, for some, not at all. For various reasons they would prefer to remain anonymous. Either they think it best to remain anonymous or they think that their signature would ruin the balance of their work. Or maybe they think they find it still wanting and not knowing what to add, they just decide not to.