I always found this fact fascinating. Why would I attribute different levels of worth to two artifacts that were totally identical, save for their method of construction?
I didn't have a good answer to this until I read a book by aesthetician Dennis Dutton (The Art Instinct). Dutton presents a 12 factor model of art, and claims that artifacts exhibiting all 12 criteria tend to be unambiguously judged as "valid art", while artifacts that miss some but exhibit most are considered "edge cases", things that reasonable people can dispute the artistic validity of.
One of Dutton's 12 criteria is "expressive individuality". Most art objects are the products of a single person's imagination. Computer generated art tends to lack this trait, and this, I think, is why it feels less valid than human-made art.
Here's an outline of the 12-factor model: http://www.aristos.org/aris-10/dutton.htm