Why are front and aft modules with identical function not simply one module that fit in both locations?

As noted above, we see these modules as the first step to a larger system. Having two smaller modules in a single ship gives you much more customizability. Sure, you can build a pure cargo Retaliator or a pure living version… but you can also mix and match to perform multiple roles. As the number of ships that use modules increases and the number of options available goes up, this will allow for significantly more gameplay.