Because (1) nobody, and I mean no one from Ford brass all the way down to the SVT engineers have ever even hinted that was the case and (2) given the additional costs associated with such improvements from materials to tooling to process to testing, one might expect Ford to advertise the crap out of the increased strength, especially when comparisons started between the Raptor and the Ram Runner. I would think the Ford personnel on hand during the PickUpTruck testing specifically, might have mentioned reinforcements as a potential reason the Raptor might be a better choice over a Ram Runner.
No doubt stranger things have happened, but at this point if these reinforcements are real, they're the best kept secret in the Raptor world.