Because everything is attached to the frame. If you look at one of the above pics you'll see the extension is locked in and can't move either up or down in relation to the frame. It becomes an integral part of it. The bag is also attached with the weight bearing portion being the top frame cross member.
Not trying to be argumentative here Bob, but when you look at the bags with built in load lifters the weight comes from the top of the says, progressively working all the way down to the bottom. If the bag does not ride on top I don't believe you get the full advantage of functioning load lifters. When packing meat the ideal location is always close to the body running up close to the shoulder blades and ideally never higher than the shoulders themselves.
The reason I bought into the Metcalf concept was because the load lifters were integrated, just like the other "big" bags that MR offers. I packed my entire deer with it in September, and the extensions make a real difference. The weight is transferred down, all the way from the top, anchoring into the hip belt the way that it should.
If a new generation frame were made that had integrated load lifters at say a 25 or 26 inch frame(similar to the tactiplane I used this past weekend), where interchangeable bags were hung off the top of frame it would be great.