I know that in certain countries like Japan or South Korea: it's normal to leave the front door unlocked

Can't speak to the Koreans. I don't know anybody in Japan who doesn't lock their door even when they're home. Granted, I don't know the entire population. Even in the countryside I suspect more people will lock when they're not at home. There is a tradition where the area immediately behind the front door where you take your shoes off is considered not that private. They call it genkan. And delivery people may - in rural areas - enter that area without being invited. The fear of one day finding the public broadcaster TV fee collector, the yoghurt mafia, Jehovah's witnesses, or the Mount Fuji cult recruiters in your genkan, makes most people lock.