Yes they are stollen due to house break ins and taking the keys, to say you won't even leave it in a supermarket carpark to do the shopping is just OTT
For this reason I always take my keys with me if I am not at home and the spare is not kept in the house anyway. Its easily accessible to me, but would be impossible to access the car using one of those relay kits to steal Keyless cars since it's so far away. At night the key I use is in the gun safe. Its an inch thick of steel. Doubt the relay signals would get through that. And if I am opening the gun safe for a violent thief... either the gun is coming out too, or I'll be pressing the panic button while I'm at it which goes through to my local police station just over a mile away.
As mentioned above, "not everyone has a garage", well not everyone has a gun safe either.
For reference, I am fully ware of self defence laws and gun control laws. And as stated in another post on home security. A prospective car thief would have to go some effort to get to a point that I'd need to be openning the gun safe.