What? I'm not sure I understand your question. Your tires are rated for a total of 15,000 lbs. I'm guessing your GVWR is substantially less than 15,000 lbs. Are you asking why your GVWR is less than the 15,000 tire weight rating?
There are many factors to achieving your GVWR, to include tires, brakes, engine, transmission, frame, suspension, rear axle, cooling system, etc.
Just because one of the factors (tires) has plenty of room to spare doesn't mean that ALL of the factors have that much room to spare
Either that or I am completely misunderstanding your question...