Because, by definition, a backup is not in the computer
In order to qualify as a backup, it must be stored somewhere other than the computer. In fact, it is recommended that critical backups be stored completely off-site (at someone else's house, for example). You can back up your computer automatically, but it is going to cost you something. Companies like Carbonite don't just offer their services for free. Even then, the system is not going to be backed up and neither will very large files. If you want it done right, you are going to have to oversee it yourself. If you want something important done, then do it. Don't expect it to be automated for you.