Why are Superbirds worth so much?

how may have you seen in real life..... I have seen a few at carlisle....and a few in binghamton ny a few years ago.....

When I was a kid back in Soo Ont in the 80's. There was a guy named Art that had one with large Plymouth signs on each quarter and a snorkle hood scoop. We used to ride our bikes to go fishing past his place and stopped in as a couple early teen kids on bikes and he was happy to show us the car and tell us about it. We stopped in a couple times and then he started to restore it. I didn't see it for a few years and it came out looking 100% stock and beautiful. I don't know what happened to that car.