It's the system we have to work with... I prefer not to make my own systems anymore.
54th century spacecraft are not 20th century jet fighters... Even jet fighters would not carry cannon if it made them thrice as large and expensive.
That said, spacecraft can carry "smaller weapons" (HG, p38) without consuming firmpoints, E.g. a Gauss Cannon, doing 1D or 2D at Adjacent range. You even get an attack bonus (but no damage from Effect).
Missile launches are still actions that require a Gunner.
However, the Gunner skill of the Traveller(s) that fired the salvo is not used as a DM.