[bombcar]

Locked doors can help prevent the door from flying open in an accident, too.

[plugin-baby]

And maybe make rescue more difficult after a crash?

[deschutes]

Doors are part of the structure of the vehicle and must stay closed to do their job in a collision. To my understanding this is the primary if not sole reason doors lock after a few seconds in motion.

[BenjiWiebe]

Not much more difficult. In case of a crash first responders break the window and unlock the door.