Why do drs use drugs to lower testosterone in cancer when bonifide studies show just the opposite to be true?

Myth of T. Drugs (or surgery) is used to reduce testosterone in prostate cancers as it can encourage growth of the tumor. You may be referring to studies (now disproven) that found increase risk of prostate cancer in men with low testosterone. These men need testosterone therapy in contrast to those who have already developed prostrate cancer.
Anti-androgen Meds. The only major cancer where a drug is used to lower testosterone is prostate cancer. Prostate cancer is fed by or sensitive to testosterone and methods to lower testosterone(surgically or with medication) results in shrinkage of the cancer. I am not aware of a "bonifide" study that shows any major exception to this.