The reason might be that most current MMO's just do not allow for meaningful RP to take place. This RP is reduced to fooling around and silly emote battles. I can totally understand your disdain for this.

However if you play in a sandbox like UO or SWG you will find that RP in those games has the potential to be fun and meaningful because the game gives you the right tools such as being able to declare war on enemy guilds, to have guild housing for socialising, offering space in player towns to hang out undisturbed or simple being able to drop items onto the ground.