For what reason$($s$)$ do some security professionals consider insiders more dangerous than outside intruders?

Employees $($insiders$)$ cannot be corrupted by criminals and other organizations.

Employees are more willing to work in groups to consolidate their hacking resources.

Insiders have knowledge of the security systems in place and are better able to avoid detection

Insiders have the most technical knowledge needed to cause immediate damage to the organization.