Deptford;
Missiles are largely irrelevant to this discussion. They travel at supersonic speeds, and torpedoes don’t. They travel through a consistent medium, and torpedoes don’t. They can be spotted hundreds of miles away, and torpedoes can’t. Their paths can generally be predicted quite accurately, and torpedos’ can’t. There are other differences, as well.
It’s true that torpedoes will give you a small active return and a larger passive return, as you’ve noted. But that actually works against your ATT. Because unless you heard the incoming torp when it was at great distance, your ATT is probably going to be moving at full speed. That means that it can’t use a passive seeker to find its target. It’ll have to go active. So your ATT will be forced to ignore all that noise the target is making, and try to find its small active return instead.
There are other challenges. Like the fact that your ATT has to be able to find targets, even though you didn’t lock onto them at firing. Once in the water, it has to discriminate among whatever objects it sees out there, including surface ships, submarines, debris, decoys, and marine life to locate the torpedo(s), prioritize them if there are several, track them, work out an intercept solution, close the distance, and make the kill. All in the time it takes two weapons to cross a few thousand yards at full speed.
And why does the ATT have to do all of these things autonomously? Because you can’t help it out. You went to flank speed when you launched, so your passive array is deaf and your active is pointed in the wrong direction. You can’t help the ATT find the fast little metal tube. And you have to merely trust that it will NOT find the fast BIG metal tube by mistake.
But none of those issues are the really tricky ones. Acquisition is straight-forward, if difficult. Target discrimination is software. What’s hard is closing in and making the kill. The attacking torp’s route is not necessarily predictable, because some torpedoes can be “flown” by hand. If your ATT is going against one of these, it could compute an intercept course and execute it, only to find that the torp zigged somewhere else. (Plus the points Sergio made.)
So that’s kind of an abridged oversimplification of the situation.
I would like to return to the subject of missiles briefly, though. I think you’re under the impression that anti-missile defense is something we’re good at. I hope one day we are, but that day is not at hand. The only system we currently have that I would call “reliable” is the ship-mounted close-in weapons system. It’s a last line of defense that shoots a stream of bullets at incoming missiles, hoping one hits. It often does, but you can see how different this system is from the torp/ATT system under discussion.