There are guys who have hoarded lots of stuff away for decades. I think there is an illusion that Mopar parts are scarce, but I think many of the parts still do exist only they are in the hands of a small number of people.

Here are the assumptions I have made after talking to a few of these hoarder type people in person:

Many think they will use it all someday. Nobody really needs seven core 440 engines if they were honest.
Some do it with the sole purpose to make money reselling it later. Usually the prices are so high they get stuck with it.
Some just like holding onto it as they like having the stuff around even though they really don't need it. It's a I found it first and you didn't story that they like to tell everyone.