Why isn't the Purgator-1 a grenade launcher?

Everything about it says 'explosive weapon'. It looks like a grenade launcher, the enemies use it as a grenade launcher of sorts, the stats suggest it should be a grenade launcher. 1.5 fire rate, 9 rounds in that massive magazine with a tiny ammo pool, no multishot, non-hitscan with a relatively heavy arc. And yet... it's just a rifle. Not even a shotgun, it uses rifle mods. Why? It doesn't make sense.