There are quite a number of packages that actually do this. I'm working on an application at the moment where I had this same thought. It would be a lot easier for settings to be managed and stored in a database.
I think it's not something that is done "as standard" as it completely depends on the application and it's purpose. For a lot of applications it would probably be overkill to put something like this in. The application I'm working on may be open sourced at some point, so settings that can be configured in db would make it a lot easier for other people to manage.
Rather than using any of the packagers that do exist I've kept it very simple. I've created a Settings model, migration, seeder and service provider.
The migration only has a few columns, id, name, value and timestamps.
The seeder contains a simple array of default values for development purposes (before I create an install script).
The model contains 2 method, I've over written the create method and am serialising the value before it gets added to the db. I also have a static get method that accepts the key you want to get and returns the value unserialised. I've also added a cachable trait to from this package.
As for getting the settings, I've registered a SettingsServiceProvider that goes through replacing the config value with that from the db. This is pretty quick as it gets the settings from cache rather than db.
Hope this helps.