It's the nostalgia of being a gateway RPG for a lot of people, combined with the incredibly high quality content of the game itself. One of the greatest things which I feel often gets overlooked is merely the depth of the Planet. You explore this world in a lot more depth than you do in a lot of games even today. Social issues, economic woes, political impact... You get to see how the people shape the world, the impact that they and their decisions have on each other. You see pointless small acts of heroism, as well as random acts of jackassery. It all just adds up to being an incredible immersive world that people just want to learn more about. And few games have offered one to compete with FFVII on that scale.