First off the trailer connection point is not 5ft. behind the rear axle, it’s over or forward of them(fifth wheel). Second the trailer axles are very close to the rear of the trailer, the trailer COG is far forward from them when loaded. Third the rear tires be it 4 duals or 4 super singles produce a large amount of cornering stiffness (grip). Even at that there’s still plenty tractor/trailer combos that still have stability issues and/or jackknife everyday on our highways. It’s rare to see a semi trailer sway but divergent (static) instability is common.