Oncologists are constantly in the business of treatment: chemo regmins, mitigation of chemo side effects, blood transfusions. Oncologists are usually consulted when cancer or hematological problems are suspected or diagnosed (usually "diagnosed" by a pathologist--not an family practitioner or oncologist).

Cardiologists treat existing health problems--congestive heart failure, aortic insufficiency, coronary heart disease--with drugs or procedures.

Sure they do also provide education, but so as to help keep patients for getting worse than they already are, unlike FPs who often are in the business of keeping healthy people healthy among other things.