A very common question in canning: If my product is already cooked, why does it need such a long processing time?
The answer is density. The processing time is determined by how thick the product is and how long it takes for the heat to penetrate to the center of the product inside the jar. It doesn't matter if the product is raw or cooked; it requires the same processing time. Those beef chunks are just as dense cooked as they are raw, Margaret. The whole point of the canning process is to sterilize both the food and the jar.
It's the same reason processing in half gallon jars is no longer recommended. The jar size is so large and it takes too long to penetrate enough heat to the center of the product. Not to mention, canning pots don't really come that large anymore unless you're a restaurant.
There is this additional benefit to the processing time: Extra gases are eliminated from the jar while it is heating through. (This is why you can't crank the lids down super tight. If you do, they will buckle trying to exhaust the gases.) After processing, the lower air pressure inside the jar vs the higher air pressure outside the jar creates the vacuum seal. The higher air pressure pushes the lid down and the low air pressure inside the jar holds it. This vacuum seal keeps additional germies out long term. That's why when you open a properly sealed jar, you hear the whoosh of outside air entering the jar.
Once this process is understood, it opens up a whole world of foods to be canned. Recommended or "tested" recipes or not. We will discuss that another day. Happy Canning!