Yes, you could mount railguns or other pnumatic weapons on the planes of Inhuman. Ships (large ships, larger than Koji's Jester X) DO in fact have pnumatic weapons mounted on them. But planes ram eachother. The reason is it is just not cost effective to mount weaponry on the outside of a small ship that can potentially be damaged in a vacuum and require that small, possibly untrained crew to have to repair it. Planes are not that huge. They are not built to accomodate something of that size or mass and the recoil of firing a gun in space- any gun- where there is no friction to slow you down means that you go flying backwards and temporarily out of control with each shot. Planes are not staffed by large groups. The hulls are built to take the impact of asteroids and space debris to begin with. Crew members are trained to repair them from the start.
Now let me rewrite that paragraph so the ones still whining might understand it.
Yes, you can mount a flamethrower on a f14. You could also mount an anti-aircraft gun on it. Aircraft carriers DO have anti-aircraft guns. But it is not cost effective to mount an anti-aircraft gun on a f14 because the entire plane would have to be rebuilt to accomodate the mass, weight and recoil of such a weapon. Besides that, the f14 pilot is not trained to fire an anti-aircraft gun. If something malfunctions with it, then it's just a huge piece of dead weight. His plane is not that huge. If something goes wrong, he goes down, especially in a dogfight. His plane is built to take some dogfighting and to fire bombs. The pilot is trained to fly his plane for that purpose.
Guns are a lot more complicated to fix than a hole is to patch.
A large proper ship can staff members, tools, resources and materials needed to maintain their weaponry. Planes can not. A pnumatic gun inside a proper ship can be accomodated in such a way that its recoil can be absorbed inside the ship. A plane is too small to do this.
There are also the jump gates to think about. As Koji explained way back in arc one, weapons going through a jump gate are dangerous due to their potential to be reconstructed incorrectly. This is a risk all ships take, that something could go wrong, but in general pilots are just not interested in having one more way they can blow up on board if they go through the gates. Jump gates are built for the use of small to medium planes who can not make long journeys on their own. Ships don't use jump gates and so they can have guns without worrying about their exploding.
Do some pirates or other fringe groups outfit planes with guns? Yes. Just like someone might mount a machine gun on the top of their car in some crazy Mad Max future. But this is not the norm nor is it safe. Safety regulations forbid guns on ships and since all legitimately lisenced pilots must have their planes undergo inspection MUCH LIKE A LISENCED CAR DRIVER they can't really SNEAK guns on there.
So here are your reasons:
It's not safe.
It's not effective.
It's not allowed.
It's not done.
No buts. This conversation is over. THE END.