Because when people see that piece of artwork again in the future, they'll recognize it more immediately and studies have shown that people are more inclined to buy something they recognize. This is a basic concept taught in advertising.

I understand the value of giving someone the artist's name, my sarcastic little friend. I'm just saying that it's more useful to the artist, even if someone doesn't know their name, for their artwork to be displayed in a small and harmless manner.