Generally it seems that some individuals feel that their personal judgement/risk assessment on whether their item is "safe" is all that is required to take it with them.
... when much of the process actually relies on pax being upfront and actually declaring their items. Every check in process (including self check in) clearly asks you to declare DGs or security items.
People aren't going to declare their items if they think they are 'safe'. When asked the DG's/security questions, they will simply answer 'no', as they don't think they have anything dangerous with them. Almost no-one will take the time to read or listen to the question properly & then consider what they have with them in light of the information provided with the question.
The whole system falls down right there!