I'll explain. The frame rails need to be strong, especially in the center line between the axles where the joint is. To have that joint in that spot would need some seriously heavy steel in the hardware, especially when you consider twisting on uneven terrain. Also, the drive shaft runs through there. Furthermore, how does the bed slide forward with a load on it?

This joint just adds needless complexity. Ramps stowed in the back end of the truck bed are best.