They don’t do mastectomies on all women because it turns out that you can identify groups of women where the mastectomy helps and others where it provides no benefit.
Breast cancer doesn’t kill you by growing into the chest wall, now it is very uncommon to see that. The problem is that the cancer cells get into the blood stream and move to far points in the body. Unfortunately, this can happen when the tumor is still quite small and apparently contained in the breast tissue. In this case, removing the breast does nothing for long term survival but removing the tumor (lumpectomy) does improve short term survival.
Thank you for that answer, that makes sense. I am not afraid of much, but cancer? Yes. Man, I hope I just have a nice, quiet heart-attack at 72. In my sleep.