It comes down to the bizarre perception that in their original forms, older works somehow just aren't 'good enough' for a modern audience.

Part of that has to do with the inclination to think that newer is automatically better, and part of it is an inability to distinguish between the degradation of source materials over time vs. whatever flaws it may or may not have had when it was first produced.

A lot of the blame for this rests squarely on Lucas himself. By creating the special edition in the way he did, he opened the door for others to follow. He's the one who first put the thought of changing films out there in the first place, and since ideas are pretty much impossible to eradicate, this way of thinking has spread to an alarming degree.