Xpantastic!

I think the answer hides in physics in that sense that the chips are not only designed for 
music, but they happen to produce harmonics and noise that comes naturally, i.e., non-
intended from the electron movements in the analog chips. All software synths are digital 
and thus discontinuous, on the contrary. And, forgetting the analog amplifiers and 
loadspeakers, they can only reproduce what has been programmed, without variation and 
individuality. Best software synths are quite near approximations of what can be measured 
and thus imitated. I don't think the sound of analog synths are a result of just a good pair of 
ears and continuous analog circuitry, but their sound is also a result of some lucky natural 
phenomenons, which all we may still not be able to measure and reproduce exactly with a 
software. This is how I see it.