Mostly because that's something you'd see far more in an offline action game. MMOs are a different beast in terms of game design that a system like that simply wouldn't work well in it. Companies have tried but you can only do so much with a free flow combat system while maintaining an optimized game.

It's not the system, it's the technical mechanics behind it that wouldn't make it an MMO anymore since it would have to drop the "Massively Multiplayer" part to get it to work right, especially outside of PvP lol.