Why torpedoes threaten ships differently from shells

Why do warships seem more vulnerable to torpedoes than to shellfire?

Torpedoes attack from below and make flooding holes directly, while shell damage more often works from the top down.

Shell damage generally works from the top down, while torpedo damage works from the bottom up. Ships can sink through uncontrolled fire or water entering through hull damage. Torpedoes directly produce the latter problem by making holes that admit water.