Because, when people talk about Final Fantasy in general, they have fond memories of various iterations. When I was in high school, for example, FF3 (stateside) was my absolute favorite of the series. But I evolved as the series did until FF7, when it simply ceased to improve upon itself. At that point I said to myself "This game is one of the greatest I will ever play". Not because I had to but simply because it deserved the praise. I said this many times throughout the course of the game. I had never experienced a sense of true emotional response to a video game other than frustration until the Aerith's death scene. And I had never known that RPGs could be more than one dimensional products on consoles until this game.
It's the only game I've ever played that embodies morality, theology and psychology the way that this game does. Everything from Cloud's identity crisis to the love triangle between cloud, Aerith and Tifa to the struggle between sense of self-preservation versus acting on behalf of humanity and, to a greater extent, religious versus scientific faith. It hasn't been portrayed that well in the gaming world since FF7.
And how many villians still rank among the best if not THE best 25 years after their inception? Can you name any other than Sephiroth?