This question has been asked to the Devs before and they're always stupidly obtuse about it, saying they want to "preserve class fantasy" or "it would look strange having a white mage use a warrior axe" (actual response despite no one asking for weapons to be glamed like that). The real reason they don't do this is likely simple, if they made glams cross role, they wouldnt be able to repackage gear models but with different class restrictions as loot.