The reason they're worth so much is the same reason an original Duster is worth so much - they literally rotted away.
I think after the Bronco II came out, I can remember seeing 1 original on the road. And it wasn't really "on the road," but the field vehicle of a guy down the road from us. The only reason he had it is he worked for Ford Assembly in Cleveland and got it cheap before he retired, and it didn't go anywhere.
Road salt was the death of many vehicles. Those down here in Texas ain’t in that bad of shape. Even ones that beat in a field for years.
Those cars here in Ohio lasted for a few years. At best.
I briefly had a 73 Duster. It had 50k on it, original spare in the trunk. A year later, it was essentially unable to be sat in because the car was a thin skin of paint on rust.