Look at the lore, look at your argument. Yes you could force some reason for titans to have a ranged super. But its not them, they are the cqb class and their supers support that. They are not supposed to be long range masters so they don't get that super. The wall in story doesn't need mortars it needs people in it keeping the fallen out. Yes archers are useful for defense, that's why hunters help out. But the primary job is on the titans shoulders, so they are set up to do the job the best.