chequers wrote:It answers the question from a game mechanics standpoint. If you are after a lore-wise answer, I propose "because the trap contains an evil spirit. When the trap is disturbed, the spirit seeks out the closest dungeon invader and debuffs them".
Actually, it doesn't answer the question from a game mechanics standpoint and claiming otherwise is demonstrably false. Consider the following potential states:
- Monster stepping on trap impacts player
- Monster stepping on trap does not impact player
In both of these cases, the trap activating is or at least can be harmful to the player.
The former makes the trap harmful to the player with more frequency. njvack made no arguments considering relative frequency, player positioning/thought process when interacting with traps contingent on monster actions, and certainly didn't acknowledge that other traps break said design statement in some situations (shafts and teleport traps can be outright helpful).
Instead, the statement was just "Because traps are designed to be harmful to the player.". Since "traps are harmful" is (mostly?) true regardless of the state posited by the OP, it's a non-answer at best. Similar in utility to saying "the trap triggers because you can train spellcasting".
And as svendre points out, even alarm traps can be beneficial. There are times where intentionally triggering one makes clearing the floor safer, hence the "at best" above.