There must be a reason...

[Del Murder]

I think for me it's the fact that FF usually has the most complete experience, so to speak. It'll usually have a fairly engaging story that's funny and serious and everything in-between, along with being fairly cinematic, an enjoyable cast of characters, a fairly engaging combat system, an interesting world, etc. I feel there are many series that handle one or more of these aspects better than FF, but with Final Fantasy you kinda get a little bit of everything and that's what makes it feel satisfying.

I agree with Fynn, it's about the complete experience FF brings. The games aren't the best in every area but they don't have a lot of weak points. I also think they have a lot of replay value as for most of the games you can make different decisions to change your experience. They also have really good pacing and don't hold your hand for too much of it or have long periods of downtime where you either have to grind or the story doesn't go anywhere.