Yeah that explain why she sent forces into Arathi and the Hinterlands, not invading other regions at all, no sir.
As to why she did not attack the Argents? One that would mean the Horde being force to choose between supporting the Forsaken or waging war with the Argent Crusade, the Alliance and likely the Cenarius Circle as well. Two: The Argent have weapons designed for killing the undead. I don't think the dark lady want to test her luck against the Ashbringer.