Fighters need to be fast and nimble, climb fast, be hard to detect, all that. Armour negates that.
Attack Helicopters and close air support planes need to be next to the ground where there’s lots of small arms. They need armour, their speed is important but not everything.
Bombers… well old style bombers had armour against machine guns and weapons to fight off fighters, current bombers not really as a missile will do you in regardless. You need also speed and being unseen.
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