CD players might not deliver all the bits to the computer accurately. The CD being recorded might be dirty. Or the sampling frequency of the recorder may not equal that of the player. The laser in either the CD player or the CD writer may be slightly misaligned. Then there's the whole mysterious problem of jitter, the doubling up or skipping over of samples that are extracted from an audio CD, a problem still fairly prevalent in CD recorders.
Digital copies are far from perfect.
Among all the compression methods, software glitches, differing sampling rates, substandard hardware, file formats, successive copying, and other factors, digital recordings clearly diminish in quality as they are passed from person to person. This won't stop people from making mix CDs for their friends, or passing songs around the Net, of course. But sooner or later, music fans, always a picky lot, will return to the source to purchase the original music in its full window-rattling clarity.