Heat breaks down the food. Nutrition is lost in the process. Even if all bacteria were killed, the fact that the food has been heated, slowly cooled then heated again means that a lot of the nutritional value of the food has already been lost.
Nutrition killers... heat, oxygen, water. The shelf life of the reheated canned food would be too short to warrant the effort. Fresh foods exposed to heat for as short a period as possible to kill bacteria then canned. It's the only way to do it.