Because a common tactic kids are using ATM is a simple snatch and grab, right out of peoples hands. Heaps easier than taking a laptop as you might be standing in a crowd, they can approach you with speed, shunt you off balance and are gone.

Phone doesn't lock while they run with it as they are touching the screen, they get away from the user and disable the security features. You are compromised, phone is compromised, thief's win.

Or so the papers say, and several female friends who have had this happen to them at evening events (not identity theft, but the snatch and grab).