Mehrunes likely gained considerable power as a direct result of Tharn's actions and other events preceding the Oblivion Crisis, and then the actions of mortal worshippers that caused the crisis itself brought him to a level where that was necessary. If he had enough power to pull those kinds of stunts all the time, why would he need the Oblivion Crisis and all the preceding strife of Uriel Septim VII's reign?
I think the real problem is Molag Bal lacks ambition for anything that grand. Meanwhile, Mehrunes Dagon is the Daedric Prince of Ambition.