Why will my tissue aortic valve replacement only last for 7-12 years, especially because my original lasted 65 years?

Foreign material: A tissue (pig or cow) or "bioprosthetic"(synthesized from natural tissue such as pericardium) generally lasts 7-12 years(with some variability) because, unlike your native valve, is foreign to the body. It is susceptible to certain stresses that your own valve wasn't. They are safe, however, and improve the quality of life during those years.

Which type of tissue: If your original lasted 65years you should be older than 40years old. Need to clarify.

It's not alive: The tissue valves we implant are made of treated tissue from either a pig or cow. They are not living tissue any more. So over time they begin to wear out. On the other hand you original valve is alive and can repair itself to some effect.