RPGs are the games I want to play. It's that simple. I think every game designer's dream is to make the games they would want to play. They have a desire to play types of games and maybe not enough of those types of games are being made, and so they take it upon themselves to make those games.
Really, the more important question becomes, why do I like RPGs over other genres? Story is a very important part of an RPG for me, but ultimately, I know I can just go read a book if I want an engaging and emotional story. It's the gameplay of RPGs that glued me to the genre and kept me hungering for more. I'm obsessed with stats -- I'm always examining the stat gains I receive each time I level up in games, and I LOVE comparing stats between party members. I LOVE examining an RPG's character progression system and figuring out how it works. I love seeing the damage I deal increase as I progress through a game. These are all things that give me enjoyment in a game's game play systems, and they are mostly non-existent in other genres. Sure, more and more games are incorporating RPG-like systems in them, but if they only take the high-end concept, and don't go in-depth like RPGs do, then it really doesn't have much of an effect on me. For example, in Darksiders, your weapons can level up two or three times (can't remember exactly how many) but they level up very slowly over time, and the fact there's only two or three stages to the weapon means its really an insignificant portion of the game. It does not attract me the way it would if the weapon leveling was a main focus, and could level up 10 times, or 50 times.
I can't get into Western RPGs (Bioware, Bethesda) because their games have an entirely different focus than JRPGs. Leveling up takes a back seat in those games, or at least, they have extremely low level caps, like 20 or 30 or something. I LOVE leveling up, and by reducing the importance of it is a way to reduce my long-term interest in the game. They sometimes have no "party members" and have you running around by yourself, or you are less involved in managing your party members as you might be in a JRPG. Experience points are earned primarily through quests instead of fighting monsters, another thing that really kills my enjoyment.
So I know what I like about the RPGs I play, and I know what I hate about them. I make RPGs so that I can try and go berserk with the systems and mechanics I love so much, while making sure not to include the elements of RPGs that don't "gel" with me. Sometimes I'm inspired by an awesome mechanic in an RPG that just wasn't executed very well (the Additions mechanic in Legend of Dragoon), and I have a desire to try and execute that mechanic "properly", or at least, in a way that I would be satisfied with that mechanic as a player.
RPGs are not what got me into games (Platformers were my first love in video games), but they are what made me want to make them.
