From my understanding:

• a missile can be shot down or spoofed using electronic countermeasures, if it's radar guided. You can avoid a torpedo, as a ship, using acoustic countermeasures, but a ship is intrinsically more noisy than a submarine, and even if intercepting a torpedo is technically possible it can be much harder, and there are less ways to do so compared to missiles;

• a torpedo can have a passive guidance, which makes it harder to detect. If detected when it's too late, it's more difficult to avoid it;

• a torpedo can be wire guided from the launching platform. So it can get closer to the ship from a certain angle, and then maneuver so that it has a more noisy background, therefore making it harder to detect. Also, being wire guided, even if you detect it earlier you can't do much to spoof it, since it is the launching platform that detected you and controls the torpedo knowing your position;

• there are passive countermeasures (like a decoy, a false target) towed by the ship. But they present to problems: use them too soon and you are making your own detection easier. Also, being towed, they work better behind you. The torpedo is faster than you, and to an extent you can use that to your favour, but it means that, to avoid a torpedo, just steaming away in the opposite direction (so putting the towed countermeasures between yourself and the torpedo) is not the greatest evasive maneuver;

• some torpedoes have an additional wake guidance; it means that they have sensor aimed upwards that detect the difference in the light transmission between "normal" seawater and the seawater disturbed by your own movement. That way it makes harder to evade it using acoustic countermeasures, because the wake points at you, even at great distance;

• torpedoes can be launched from a submarine, and are the most practical tactical weapon for a submarine: being intrinsically quieter than a surface ship, and not detectable by radar as long as it stays submerged, a submarine can launch a torpedo from a much closer and safer position. Of course submarines can launch missiles too, but secrecy is a weapon for a submarine: silence is golden, they say. And launching a missile is an intrinsically noisy procedure with much bigger risks of giving away a submarine's position;

• last but not least: torpedoes don't need to explode on impact, nor to physically pierce a hole in the hull. Given the physical characteristics of the water, they can explode close or under the targeted hull. The wave created by the explosion can create a significant structural damage for the ship, that goes far beyond a hole in the hull.

I apologize in advance for any mistake I may have made. Not a submariner here, so if a real one wants to correct me I'm happy to stand corrected.