Why don't doctors just remove cancerous lungs instead of recommending chemo and radiation?

Depends on pathology: Some forms of lung cancer (small cell lung cancer), are best treated with chemoradiation. Others ( non-small cell lung cancer) if surgery is possible it is best to be treated with surgery, plus or minus chemoradiation. In short, pathology and extent of disease involvement dictates how the tumor should be treated.

Depends on stage: Surgery is done and recommended for early stage lung cancer. When the disease is more advanced surgery can't remove all the disease so doctors recommend radiation and chemotherapy which works better than surgery for those patients.

Stage dependent: Therapy for lung cancer depends on the stage. Tumor size and local extension, lymph node involvement, and distant spread are all taken into account. Only early stage lung cancer is treated with surgery.