But, if the value is simply as a signifier and the rules of society are just agreement, can I replace all the locks with signs that say "please don't take this" and replace the police with just expecting people to be kind all the time? Of course not. The agreement of society is meaningless if nothing actually enforces any of rules. Enforcement may not be perfect, you indeed often can get away with crime, but for most people the expected payoff from stealing their neighbors keyboard is worse than the expected payoff of just getting a keyboard the "right" way from the store.
The cost to stealing the keyboard depends on the time spent in jail if caught, times the probability of being caught. The probability of being caught goes up the longer I have to fiddle with a lock. The lock doesn't protect the keyboard per se, it merely makes the expected payoff of stealing not worth it to most people.
Most suburban homes are actually unlocked. If the front door is locked, there is a high chance you can get in from the backyard or from a window. It's easier than you think to break into a home. Most of your neighbors are working in the day, or even if they are not, they are not actively checking your home. It takes less than 2 minutes to break in.
You can also bring a van and two uniformed men and take all the possessions there is. People will think you are moving out; and not being actually robbed. When you have a higher criminal population, life become hell and no amount of policing or security will help. (ie: Some countries in South America and Africa).
> keeps honest people honest
More like keep curious people out. Most people will not enter your house even if the door is left open.