From a patient's point of view, a cure is vastly superior to a lifetime of treatment.
From a drug company's point of view, a lifetime of treatment makes more money than a cure.
Drug companies make money through doctors and hospitals. The drug companies need the doctors to dispense their products, because the doctors are writing the prescriptions. To influence the doctors hand, drug companies employ a legion of medical writers, illustrators and salespeople. They flood the doctors with samples and information on how to treat disease. The motivation probably isn't sinister, but the results can be. The drug companies cannot be given responsiblity for directing the war on disease.
Directing the fight against disease is in the hands of regular doctors and researchers. If they aren't taking orders from drug companies, and they are motivated more by healthy patients than wealth, their work will lead to cures.
This is similar to the arrangement of weapons dealers to soldiers. The weapons dealers want to influence the soldiers in a conflict, creating a greater demand for their products. Weapons dealers aren't helping to find a solution to the conflict. There is a need for someone to track down the solution, the connection, process or shortcut to end the war. The weapons dealers aren't going to find it. They aren't looking for it.
Pictured: Three days worth of Helidac Therapy, in 16 pills/day sheets.