Mabye I have missed a lot of points, but making a pure ranged DPS would be way too OP. Unless you mean like a DPS that has to stand still, like BLM without cast times?
I mean being able to move on brd/mch AND DPS while avoiding mechanics is a huge plus side the the job (Its why they made the trade-off of standing still to improve DPS). Also they have zero positional requirements so all attacks always land for full potency. All pure DPS (melee) have little nuances that make them more difficult to always attain maximum potency.

TLDR: being able to avoid all mechanics while constantly DPSing, no positionals, allowing hitting for full potency would be too OP.

maybe I just named the obvious though.