I'm afraid that the premise of your question is fairly flawed. Artists from Michelangelo to David to Picasso to Koons were (are) quite famous during their lifetimes. Art history is possessed of a few outstanding examples of artists who toiled in relative obscurity before being discovered after their deaths, the most famous of whom is probably Vincent Van Gogh. I would also add a third category, artists who were famous in their time, but because of changes in artistic tastes fell into relative obscurity before being rediscovered by later generations. But it is inaccurate to say that most artists were not famous during their lifetimes.