Very little research currently goes into attacking aging directly - as opposed to handling things that are in no small part downstream from aging, such as heart disease. A big reason for poor "longevity gains" is lack of trying.
Age-related illnesses shouldn't be dismissed with "they're just old" of course but there's no reason to expect a single cause. Other than passage of time itself.
That's not a reason to say that cancer is somehow "not a disease". It obviously is. We don't want cancer. We fund efforts to research cancer and funnel money into better cancer treatments, and we get results.
Aging should get the same treatment.