because axle tubes and tires are typically the "weak link" in a trailer, and many modern trucks. They were spec'ed such that they are being used to their fullest at a full load, so you'll get your money's worth out of them. Deuce tires and axles are over-spec'ed for the truck that they're installed on. Look back a couple of posts at yeager1's picture. even with tandem axles you've still got a lot of unsupported frame there, you just can't "see" how unsupported it is with all the empty space under it filled up. The frame rails can only carry just so much. Every part, the axles, wheels, tires, lug nuts, springs, spring shackles, spring bolts, spring pin bolts, the frame rails, the bed mount bolts, the bed it's self, literally every load bearing part has a weight limit. It's up to the designers to determine how "matched" they are, or if there's overkill built in somewhere. For cross country travel, extra axles and tires is good for floatation, redundancy in the event of a flat, the walking suspension smooths out bumps, extending the life of the whole vehicle, reducing operator fatigue, etc. You will loose all of that benefit for sure.