I had one couple at my yard who'd been looking for a cupboard door for two years. Passion and obsession play a part as do investment, status, and the one that isn't easy to explain - the mystery of the past. Now I know it's not always possible to find that inspirational artefact you're looking for. If what you want has disappeared then buy something that's reproduced using the same level of skill, design and material, and contains that ingredient called quality. Some salvage-oriented businesses are a victim of their own success.
Demand is increased by raising public awareness, so replacing the stock becomes harder.
They look at it from the angle, 'well, you can only sell an original once, so copying it is the way forward'. As it's an easy way to make money, outlets are springing up everywhere and many have no experience of old shapes. China is mass producing all kinds of reproduction - good and bad. I saw a Chinese copy of a Victorian fireplace - it was the first time I'd seen cherubs with Oriental eyes. I've heard of some cracking under heat.