Why is metastatic breast cancer difficult to treat?

It's not: There are many treatments for metastatic breast cancer which can control it for years. Although it can be treated, it is rarely cured. That is because cancer cells are constantly mutating, so treatments lose effectiveness. Metastatic breast cancer requires constant treatment with either hormone therapy, chemotherapy, or biologic therapy. Treatments are changed as they lose effectiveness.

It can be treated: Metastatic breast cancer means it has spread to other parts of a person's body. At this stage, currently, it is not curable. But, definitely there are therapies of many types to hopefully control the disease. Surgical, chemotherapy, radiation, and more. It is true that some types of breast cancer are more aggressive and less treatable--only because we haven't found out what makes that type "tick".