You're way, way over simplifying the weapon balance. You have to look at the way different weapons shoot, for example the speed of rockets, their splash damage, their rate of fire, their max ammo capacity. It's actually quite complicated.
I mean look at the Biogun. It's got a complicated arc of fire, which requires a lot of skill to actually hit people with, but it has advantages like people walking over the shots.
Ultimately, it's tweaked for fun over balance, and that's what it delivers on, fun. However to say Halo is without balancing issues is a blatent lie and you know it. Not one game this generation is perfectly balanced, and the only series that does balance well on consoles is Socom. The best we've got right now is Warhawk or Shadowrun. Unfortunately, Shadowrun besides being balanced well also happens to suck balls.