There are real life precedents for this design decision. From the wikipedia page for the B-17 bomber: "Defensive armament increased from four 0.50 in (12.7 mm) machine guns and one 0.30 in (7.62 mm) nose machine gun in the B-17C, to thirteen 0.50 in (12.7 mm) machine guns in the B-17G. But because the bombers could not maneuver when attacked by fighters, and needed to be flown straight and level during their final bomb run, individual aircraft struggled to fend off a direct attack." Like most bombers in WWII, the B-17 was equipped with multiple anti-air weapons that were not very effective. The UEF/Cybran strat bomber is likely based off of this—and their lack of anti-air effectiveness is also accurate.
Battleships in real life had even more anti-aircraft weaponry that was about as ineffective as it is in the game. From the wikipedia page for the Yamato: "Yamato carried twenty-four 25-millimetre (1 in) anti-aircraft guns, primarily mounted amidships. When refitted in 1944 and 1945 for naval engagements in the South Pacific... the number of 25 mm anti-aircraft guns was increased to 162." That didn't stop her from getting torped to death though.