Another consideration that Ford takes into consideration is registration and licensing requirements. When capacities go up drivers license requirements get more complicated, and putting plates on the back of the thing gets more expensive, along with insurance etc etc. Plates for my 450 cost $250 for 1 yr.

Ford rates the axle in the back of my truck at 9500# Part of that I'm sure is because of the tires they put on it, but also with all of the above in mind. The axle manufacturer otoh rates the axle at 14,500#.

The truck you are talking about is in a whole other class and makes no bones about the market it is intended for.