Doctor Strange says that he monitors individuals and beings from other realms (planets such as Asgard or Knowhere, dimensions such as the Dark Dimension, etc), not the realms themselves. So long as the individual is not the ruler of their realm/planet, he can presumably leave their own law enforcement to monitor them when they are nowhere near Earth.
As an equivalent: the CIA or MI6 may monitor specific foreign nationals who have been flagged as potential threats, but they do not monitor everything going on in every country in the world - for one thing, they don't have the resources/manpower! Doctor Strange is thus highly unlikely to be able to monitor more locations with less resources.
Also - Doctor Strange takes over from the Ancient One after the events of "Captain America: Civil War". This means that, for example, during the events of "Avengers" or "Age of Ultron" he was still either in training, or had yet to travel to Kamar-Taj, and hardly in a position to intervene.