I think a real consideration is how fragile those early airplanes were. Machine guns are heavy, their ammo is heavy, and you need heavy mounts to absorb the rec

I think a real consideration is how fragile those early airplanes were. Machine guns are heavy, their ammo is heavy, and you need heavy mounts to absorb the recoil of firing them. The wings of WW1 fighters are basically made out of toothpicks and tissue paper (an exaggeration, but not by much). I wouldn't be surprised if they simply couldn't handle the load of wing mounted guns.