Why do artists do not sign their work?

[Amateur]

Why? I do not mean the Middle Ages or icons - where its rules. And the artists of the XX century, those-why? I can understand when a picture or sketch out little weak - so that you do not want to associate them with your name, so eliminate them, because after all, there were many. And the question does not arise. But if "race" and the product did not kill as Taras Bulba, means - recognized, and then give it to him as a father, his name ... And then roam the world thousands of unsigned paintings as abandoned children and the poor collectors are forced to spend effort and money to establish paternity, sorry, on the attribution. But seriously, what is still driven by the artists, when they do not sign their work? Because often it is not for the weak jobs ...