Because they satisfy every gaming need of purpose in a game. The characters actually make me care about the game, the stories are paced with the gameplay creating a great experience, the music draws me into it, the gameplay is simple and easy; thus never frustrating me and making me start over. You have many choices that have a lot of impact in the game; simple things like stealing or morphing enemies into items without overly complex stuff that I just don't care about.
If you're into more hardcore gameplay, some JRPG's can let you max out your stats if you feel the need to.
Multiple side quests and minigames keep you going if your brain wants something different for a period instead of the story.
That's the appeal
This will only take a moment of your time. *steals your watch*
