Because the Sorcerers are there to protect against bigger threats than just simple threats like a Chitauri invasion. They are there to protect against the mystical and other dimensions whereas people like the Avengers can handle the clear physical threats as Wong himself states.
Doctor Strange: When do you start telling me what we are?
Wong: While heroes like the Avengers protect the world from physical dangers, we sorcerers safeguard it against more mystical threats.
The Sorcerers themselves though do have to protect the Sanctums, and though Wong states below they protect them from "Other-dimensional beings" protecting them from immediate physical danger by simply destroying a few things coming nearby doesn't seem like much of a problem.
Wong: Together, the Sanctums generate a protective shield around our world.
Mordo: The Sanctums protect the world, and we sorcerers protect the Sanctums.
Doctor Strange: From what?
Wong: Other-dimensional beings that threaten our universe.
In short she is only protecting the Sanctum because the Sorcerers are only there to protect the Sanctums and threats beyond the physical.