SE's reasoning is that if you are ranged physical you have mobility and survivability over a melee. You are not subject to directional attacks and have no risk of enemies cleaving you with attacks in melee range.

Due to this, SE balances the damage output of physical ranged by lowering the overall ceiling of damage.

There has to be an incentive for someone to play melee dps, if a ranged physical can reach dps levels of a melee and not have any directionals or risk of cleaves, then there would be no point to playing melee.

TLDR - ranged physical will always have reduced damaged output comparatively to other classes, due to "mobility" according to SE.