Why cant we buy fighters

Why cant we have fighters as anti-air units that we can buy. they would help lot against SM and can catch up to bombers. we have the anti-aircraft which protects our city's and land units but they don't move fast and cant catch up to the bombers plus they get only a defense against them and then we have the destroyers and subs which get a defense bonus and help protect the sea but bombers can still get away from them why is it that we only get defense bonuses and not attack like fighters plus SM weakness is in the defense of bombers but sometimes it hard to attack them because they can get away. like when there on land they can go to sea and from sea to land to get away from trouble.

Fighters are a rare unit. There are no plans on changing default units at the moment.