By default WordPress for permalinks uses the native slugs (post names) that are created when the post is published. The native slug are not updated when the title is changed and that is why by default the permalink stays the same.
In this short article you can find more information on what exactly the native slugs are.
How to automatically update the custom permalinks?
The custom permalinks can be updated automatically after a simple set-up. You can find more info on how to make the custom permalinks always use an actual title here.