it's similar to PC - if someone full formats a drive, nothing survives. The root protection is there since a lot of thieves aren't that skilled with smartphones. Dumb thieves will only throw away the SIM card. Smarter thieves will perform a factory reset. You can't flash the ROM just somewhere on the street, you need some equipment to do that, usually an Internet connection as well. What you can also do is root your device, install Anti-Theft rooted, then unroot it back. The thief would then need to root it first to be able to at least try to remove the Anti-Theft.