I would say an insider is already having an edge over an external attacker in that he is already within the perimeter; he is not struggling with external defense, he is trusted and has earned the trust of the organization and accepted as 'inside'. From a moral and technical point of view, the vulnerability of an organization begins from 'who do you trust?' (Carl Colwill: Human factors in information security: The insider threat - Who can you trust these days? 2009).

Also, an insider already knows the weaknesses of an organization; he may take advantage of vulnerabilities of the softwares the organization is using and so many examples.

Convergent groupthink

short corporate memory

recruitment inadequacies

why.....?

I just want to get different perspectives and understandings

Thanks

Ahhhh. If you explore the literature of management and group personal dynamics you will find lots. Companies/groups/organisations go through life cycles like people , and possible answers to your question also need to include an understanding of where the organisation is. Applies to the dynamics of all people-containing entities.....