Tailgate problems of the 2005 GMC Canyon

I took my truck to a local shop to have the fuel system looked at after I started smelling fuel. The service manager calls me that evening and request I look at the truck while it is on the lift so I can see the frame for myself as the fuel system would be the least of my concerns once I saw the frame. The rear of the frame from the rear spring hanger back is rotted away on a 12-year-old truck that has no body rust and some rust on the other parts of the frame as to be expected for the age. I have attached pictures of the spring hanger detached from the frame due to the lack of structural support and pictures of the bad frame section and the rest of the frame along with a picture of the truck. I have owned the truck since it was new and taken care of it and am troubled that a structural item such as a frame only in one section, the same section, on both sides is in such bad shape that the mechanic recommended that I replace the frame. I know I live in an area that receives snow and the roads are salted but cleaned my truck frequently to wash the salt off of it but a truck with 112,500 miles that is 12 years old should not have a structural failure while all other parts including thin sheet metal are still in great shape and have not been painted. The only repainted items on the truck are the hood (aftermarket) and tailgate. So why does this truck have a rotted structural component?.