Because an unlocked door is per definition not secure. If there is just one case that unlocks without you wanting it, it’s already a risk that should not be added to a feature that is designed for “peace of mind”.
Automatic unlocking in the morning when you are at home, is something the Smart Lock can currently not do, because it does not know where you are. If something like this “last person left” feature request gets implemented, it would be possible. You could also file a feature request for the feature that you want.