My dad has lung and lymph node cancer. His doctor wants to wait for his cancer to grow more before they do chemo. Why wait?

Ask the treating doc. You are right. Waiting does not resolve the problem unless there is no good treatment available or if the treatment side effects are greater than the benfit it offers. So ask all these questions to the treating oncologist. First you need to know the type of cancer your dad has? Is it a lung cancer or a lymph gland cancer(called lymphoma)?
I agree. I agree with dr. Legha and wanted to add that it's very helpful for you and all concerned if you pointedly ask the following question about any treatment that is offered to your father. Is this treatment aimed at a possible cure, or is it palliative? Otherwise, decisions made by your father will not be fully informed ones.