To actually answer the question of "why," every bullet in this game looks exactly the same up to naval artillery. Besides that, every projectile is a "yellow streak" scaled up or down based upon the size of the round. I believe AAA fire is made a bit longer to increase visibility. The naval artillery consists of a "chunk" of near black material.
So, really, it's just a technical aspect. I would speculate it's done that way because a video card can draw that "bar of light" easier than rendering an object. If actual balls were modeled, your frame rate would hit the toilet as soon as someone opened up with .303s.