"Against Covenant shield technology, the rounds possess enough kinetic energy to punch through shields, cut through the ship, and, upon exit, still retain enough energy to destroy a second ship, and cripple a third ship." Thats why. While a laser could potentially deliver the same amount of energy, it would not punch through so much as vaporize the top layers and have the rest of the energy scattered in the vapor. Accelerating a missile to anything close to 4% C would crush any kind of guidance built in due to extreme g-load, assuming it takes less than a second for the projectile to leave the weapon its 0 m/s to 11991698.32 m/s in an instant. if someone would like to calculate the g-force of such a launch i would be interested in seeing the result.