It’s important not to confuse the terms “placebo” and “control.”
Control = group not receiving experimental treatment (but could still receive some sort of treatment such as standard of care)
Placebo = specific type of control which receives a dummy treatment (rather than standard of care)
For cancer, if there is a standard treatment available then that treatment will be used as the control.
It would be hard to imagine the FDA or hospital investigational review board (IRB) approving a strict placebo control in that situation because it would be unethical to deny a patient the standard treatment.
Where I believe that placebos can be used for cancer are in cases of rare cancers where no treatment exists or of late stage cancers where all other treatment options have been exhausted.
In those situations the patient wouldn’t have received any treatment regardless so there is no denial of care.
(There are some additional nuances where a placebo may be used as an adjunct to standard treatment, or where the patient could cross-over.)