Can't believe this is still a "thing".
I'm going to educate some of you now so prepare yourselves.
What you're seeing is a truck with an open c-channel frame. I know Ford, for example, made a huge deal way back when this vid, etc was going around about having a closed box frame. Hmm, guess what. Just so happens Ford's heavy duty has an open c-frame, as well. Guess what else. It's for the same reason Toyota uses one. For heavy duty use. Have any of you ever heard of real issue with Toyota's frame during heavy duty use? Have any of you ever ridden on a Ford, Chevy, AND Toyota and full load while towing? The Toyota will move much smoother with the c-frame, much more controlled and unlike the closed box under super heavy duress, the frame is likely to crack or be permanently damaged.