To answer the OP: because ER's camper is a substantially heavy (some might say Bismarck or Tirpitz heavy) vehicle and it only makes sense to go with Ford's highest weight rating "pickup truck" design.
Depending on the year, the Dana 110 rear axle is rated to ~14k lbs. Most ER's have a rear axle weight in the 11.5-13.5k lbs range, loaded with 90 gallons fuel/90 gallons fresh water and don't forget the 300-400lbs of batteries, grey/black water tanks, real maple interior carpentry, and then all the good stuff you pack to take your trip, etc. The load on the front axle is going to be about 6k lbs. I suspect that like GVWR, the axle rating is based on at least a few design/system constants. In other words, the companies building axles and trucks set the weight rating of their gear and once a custom company or individual starts changing things, e.g. all of the ER modifications to the base F550, the "rating" probably changes as well but it takes a rare breed of dedicated individual to go through all of the trouble to get an OEM to certify an increased GVWR. I would think the standard loadout on a typical overland-expedition Toyota Taco heading out for week or two in the bush is probably exceeding the GVWR quite a bit...hence all of the suspension, wheel, tire, engine, etc. mods. Everyone seems to be doing fine.