Apparently, every key is different, just enough to make the cars know what key is its own.
Repo guys have master keys. I believe the master key they have is a Slim Jim, backed up by a Tow Truck. I have seen them on shows with a special key that lets them drive the car away....
You would think auto theft would be rampant, and in some areas it is - but not due to keys. Truth is, some people just dont want to spend the jail and court time driving cars that arent theirs.
I have put my keys in other peoples cars before to test this, and they never opened.
I have heard stories of old GM cars from the 80s (Delta 88, etc.) opening each other's cars. They probably did. Cars took a long way to come to where they are today, and some just didnt get it right.
Not really basically there a thousands of combinations for a vehicle and if many of those vehicles are around its possible that you could use a key for two identical vehicles. you would just have to be lucky enough to find one like that. that's why vehicles now have a chip so that even though the key fits it still wont start since the chip is not programmed to work with that vehicle.