Why can I not do my cardiac rehab on my own?

You need to be watch. In cardiac rehab you are constantly being monitored with your heart rate and blood pressure. After a cardiac event, either heart attack or surgery, you are the most suspectiple time to have something go wrong. Don't do it on your own.
Specialized service. Cardiac rehab is a structured program run by medical professionals that includes supervised exercises tailored to your medical and physical condition, education on how to control your risk factors for heart disease. Multiple scientific studies have shown it improve patient outcomes like quality of life, survival, reduced re-hospitalization, better control of cardiac risk factors.
Self rehab. Probably you do not have the training and knowledge to do this on your own. Follow your physicians advice carefully to keep you safe.