Why Are Melee DPS Forced Into One Job Through Gear Yet Nobody Else Is?

[Ibi]

Then when NIN was introduced SE decided to make it DEX instead of STR so then that Job had to get its own gearset because thematically (and prolly stats wise too) it wouldn't be compatible with BRD gear.

There's no particular reason SE couldn't have continued to put DEX on the Striking sets though (as they went back and did with the pre-2.4 equipment) and just allowed monk and ninja to share those sets.