You say I'm completely wrong but then go on to make exactly my same point. Sure theft is possible and yes, a lot of the time the lock is merely a signifier that "taking this breaks the rules" rather than a substantial physical obstacle to breaking the rules. Most of the time people do follow the rules.
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.
Even if the door is left open, most people aren't entering your house for the same reason. If you're gainfully employed, the price of going to jail is high enough that you aren't going to start breaking into places even if you know for a fact the door is left open 90% or more of the time. The expected payoff of stealing from your neighbors isn't worth it.