Because people appreciate it by monetary value ... "ooh it's worth £12,000,000 it must be really good art" and monetary value of art is down to scarcity and excellent marketing giving rich people (and public institutions that should know better but are run by fashion snobs) something to spend unfortunately large amounts of money on.
Maybe it is more valuable as a part of the puzzle that is Lenoardo da Vinci. We never got a chance to hang out with the guy, so piecing together everything he did in his lifetime gives us some sort of context to appreciate and learn about a human genius.
The medieval artists who never signed their work would agree with you. However, our (= the modern West's) understanding of art as a form of self-expression makes the two, art and artist, inseparable.
Forgive the shallow speculation, but I can't help wondering if there's a link between your comment and the Indian background suggested by your user name. My point is that different cultures understand art differently, as they understand self differently. But I'd also bet that the modern West's conception of these things is spreading pretty rapidly everywhere. There's no modern counterpoint to it yet, only traditional ones. Probably there will be eventually.