From a med student: cancer cells need glucose to survive. Why are ca patients not receiving metformin?

They are. There are now multiple clinical trials testing metformin in cancer patients. The mechanism for why metformin may be beneficial is not entirely clear but is currently thought to be related to inhibition of the mtor pathway.
Wouldn't help. There's no reason to think taking metformin would "starve" cancer cells of glucose. In fact effective treatment of diabetes helps to improve delivery of glucose into the cells.