They are if you believe the Harvard Business Review, IBM says 60% of site data breaches were insiders

The Biggest Cybersecurity Threats Are Inside Your Company
--hbr.org

Additionally, these organisations believe insiders were the cause of a lot of site breaches:

most breaches can be traced back to [internal] human error
--dataprivacymonitor.com

50 percent of all security incidents are caused by people inside an organization
--cnbc.com

internal actors were responsible for 43% of data loss
--infosecurity-magazine.com

Most breaches involve insiders
--digitalguardian.com

Most data security threats are internal
--trendmicro.com