as a fabricator with almost 46 years of my working career dedicated to design and fabrication, i thought that as well.
however, the top made from 1"x1 1/2", 1/8" wall tubing, plus the 3/16"x1 1/2" angle on the open side at the bottom made it seem unnecessary.
i tested that "theory" by turning them on their sides, and placing the front of my caravan on them to see if they racked out of square.
i may add an "x" or a diagonal in the future.