If we can make magic bullets that are basically artificial antibodies, why has a cure for cancer not yet been developed?
This is a good question.
The first answer is that there are antibody therapeutics for cancer.
These fall into two categories which I will call therapeutic and immunotherapy (this is not typical naming but, keeping it simple they will do).
Therapeutic antibodies are designed to bind to an enzyme or signalling protein that will weaken the tumor specifically.  An example is an antibody to nutrient intake receptors (cancers eat a lot more than other cells). HER2 is a receptor that is heavily populated on breast cancer tissue.  Antibodies that inhibit HER2 activity can slow the growth of breast cancer specifically with minimal toxic effects.
There are also antibodies to proteins found only on cancer cells.  These can be specific antibodies (monoclonal antibodies) to proteins from a mutation that only the cancer cell has.