I'll hazard a guess that the reason they're done differently, is because the Melee weapons were all done by the original Avali creator, Ryuujin_Zero, back when races all had tiered melee weapons that looked different/more impressive as they went up the tier list.

By contrast, the guns were all designed as end-game gear, the things you brought into the most hazardous parts of sector-X.

Then the game changed, difficulty sectors were removed, racial themed guns (except the flamethrower and the Novakid's guns) were removed, and Ryuujin_Zero got burnt out from trying to keep up with the development of a mod that was, frankly, massive in scope.