You're more pessimistic about people than their behavior in day-to-day reality warrants. We're not worried about intrinsic goodness, nor about goodness in extreme circumstances. We're only worried about effective goodness in normal circumstances. Can you ask someone to watch your stuff? Yes, you can. Whatever the reason, the vast majority of people can be trusted in the vast majority of circumstances. There are few that will simply let someone take your stuff when you ask them to watch it for you.

  when push come to shove

Push doesn't come to shove. When someone threatens them, they may as well have threatened you. When there's a sudden flood, accident or any other unlikely incident, then you're often no worse off having someone else watch your stuff.