Any particular reason to why melee DPS gear is split over 3 different armor sets?

Ok so DRG got more armor and NIN use DEX, but that extra armor might as well have been a trait for DRG, while DEX and STR have no effect on one another.

It does give us 2 more glamour sets... If only they hadn't been locked to their respective jobs.

All in all, the way it's done just seem redundant, Tanks, Ranged, Caster and Healer shares set, so why can't melee?