What you’re suggesting is possible technically but nobody does it because the idea of having to turn your entire plane towards a missile threat to neutralize it virtually guarantees your destruction if two missiles are fired at you in rapid succession. This is not a rare occurrence. At all. Yes, from multiple directions. That’s exactly how surface-to-air systems are set up.
It becomes slightly more reasonable if you turret the gun, until you factor in the weight of the necessary ammunition to neutralize multiple threats, the turret itself, and the independent radar used for target acquisition and tracking. A B-52 could pull it off, a fighter couldn’t yet. Not without massively compromising their payload and aerodynamics.
Chaff, flares, stealth and jamming are used because they actually work in practice. Jamming is a big one that people don’t hear about much. Wonder how the B-52 is still in service? Jamming. Wonder how wild weasels do their job? Jamming.
When you see a turret like that on a bomber then you can get excited for self-guided bullets shooting down dozens of multiple incoming missiles. But tbh at the rate we’re going it’ll just be a laser instead.
Thanks have a much more robust complement of armor compared to an aircraft. They defeat the mechanism of armor penetration in munitions before impacting the armor. They still take a significant beating after the munition is "defeated". Airplanes don't have armor in the same way a tank has. Even small arms fire can take out an aircraft, generally speaking.
The automatic ballistic defense systems a tank uses would obliterate an aircraft. They are basically shaped charges that explode outwards to deflect a munition. It only works because tanks have thick armor in a dense, heavy, package.
That is an old system, mainly used by the USSR, and mainly intended to act against long piercing projectiles of depleted uranium.
It is more common to have active electronic systems. For example, one such system fires grenade-like projectiles upward that then explode in a downward direction. The timing is accurate enough that the incoming attack is hit by the shrapnel. Another system is more like a shotgun.
The more reasonable comparison is ship-based defense systems. The USA uses the Phalanx CIWS, which fires simple dumb projectiles of the same diameter as those of a typical fighter plane gun. About 100 are fired to destroy an incoming missile.
Adding an AI to a modern fighter makes it an awful lot like a flying Phalanx CIWS. You have a nearly identical gun with a nearly identical radar.
Modern ATGMs are usually subsonic; none of the protective systems on tanks are designed to counter supersonic missiles or projectiles. There's a world of difference in countering a Mach 3 missile and a TOW or Hellfire.
Incoming speed doesn't help the attacker unless it beats the sensors, which operate at the speed of light. Mach 3 missiles impact Mach 0.1 shrapnel at about Mach 3.
That helps the defender. The faster the missile goes, the harder it gets hit by defensive shrapnel.
Speed makes the engagement cycle short; instead of having maybe 2 shots at an incoming missile, you get one. And if your system is too slow, it has no chance of reacting in time. Most systems are designed to defend against missiles, but are ineffective against cannon rounds (APFDS).
It becomes slightly more reasonable if you turret the gun, until you factor in the weight of the necessary ammunition to neutralize multiple threats, the turret itself, and the independent radar used for target acquisition and tracking. A B-52 could pull it off, a fighter couldn’t yet. Not without massively compromising their payload and aerodynamics.
Chaff, flares, stealth and jamming are used because they actually work in practice. Jamming is a big one that people don’t hear about much. Wonder how the B-52 is still in service? Jamming. Wonder how wild weasels do their job? Jamming.
When you see a turret like that on a bomber then you can get excited for self-guided bullets shooting down dozens of multiple incoming missiles. But tbh at the rate we’re going it’ll just be a laser instead.