Why Is a CEX Order Book Susceptible to Insider Trading Rather than External Front-Running?

A CEX order book is private, so external front-running is impossible because outside actors cannot see the pending, market-moving orders. However, the order book is fully visible to the exchange's employees.

Insider trading becomes the primary risk because an employee with privileged access to the private order flow could trade on this non-public information before the large order executes and moves the market. This is a form of illegal front-running but executed by an internal actor.