XB1 player

If all enemies can be killed by settlement defences, how are so many people getting kidnapped from settlements?

It is a quest that you can do over and over forever or a "radiant quest" it has nothing to do with defence.

They do work, the more defense you have the less likely an attack or other event. They don't prevent them, but make them less probable. Because game logic

As has already been stated, I don't believe defenses have anything to do with it. Perhaps the victim took a stroll to pick flowers outside the walls, or perhaps the victim was sold out by another settler for one reason or another (Novac, anyone?). The circumstances of the kidnapping itself are never revealed, eh?

Its just part of the game to offer a slight variety in missions..Ive had kidnaps at sanctuary with 150+ defense..