Each case (and type of lung cancer) is different, so I can't say for certain why in this specific case, but there are times when the tumor has metastasized to the point where it cannot be excised without high risk of a fatal outcome. In many cases, chemotherapy and/or radiation is used to attempt to achieve remission, or shrink the primary tumor to a point where it can be safely excised.
You would really need to discuss the reasoning with an oncologist, and perhaps seek a second opinion if you're not comfortable with the recommendation.