Why must a bus driver check the interior of the bus before driving, even when the exterior pre-trip inspection found no defects?

AAMVA CDL Manual Section 4.1.3, p.4-1

A bus sits unattended between runs, and vandalism or accidental damage to handholds, seats, or emergency exits is not something a driver would see during the exterior walk-around alone. Catching that damage before passengers board prevents an unsafe ride and an after-the-fact liability problem.