Not sure what you think an off road trailer is supposed to look like but everyone has an opinion. Regardless, this design has been successfully used longer than you have been alive and as noted above, settled the western US when "roads" didn't exist and people moved their entire families west with all their belongings.
As far as not being "a very structurally sound shape", I don't have any idea why you would say that. Do you believe all off road trailers should be shaped like a triangle? The proof of being structurally sound is in the pictures on their website. Take a look.
