Who leaves their doors unlocked in an apartment building when going to sleep for the night? I live in outer exurbia and lock everything, not that there has ever been a problem, but people in general just aren’t as trustworthy as they once were, decades ago. It’s a real loss.
Someone whose roommate wakes him up at 2:00 a.m. because he's forgotten his key again? Someone who's hoping women will sneak in and grope him?
But seriously, I lock up, too ... although it's kind of academic in nice weather when all the windows are open.