So, basically y'all are asking something similar to: "Hey, why aren’t there more death curses messing everyone up given the Council’s history of taking down rogue sorcerers?"
And if the game designers were to answer your question, I think the answer might be something along the lines of: "Two things prevent that. One, the Wardens have gotten very good at preventing them. Two, most bad guys are too arrogant to assume anyone will ever manage to kill them. No foresight, no planning, no curse."
(YS282 sidebar, if anyone cares.)