Why is Final Fantasy 7(ps1) so sought after, and why are people willing to pay and to charge such a high price?

My husband went to get my (ps1) FF7 resurfaced at a pawn shop, and while it was being resurfaced, this other customer offer $100 for the game. Well naturally my husband refused, because its my game. Anyway I went onto www.amazon.com and the price for this game is ridiculous! I got this as a gift back in high school and it was no where near the prices the people are paying and charging! Why is this such a coveted game?