Because everybody's life is different, and everyone has different needs and different demands on their time. The purpose of the Sleep habit during the game is not necessarily to achieve a perfect amount of sleep, but to look where you might improve. All that is required is that you declare an amount of sleep that works for you and that represents an improvement in your sleep habits if you get it every night. It should be a small stretch, but it should be manageable and not stress you out.
Ultimately, there is an amount of sleep that is best for you. This habit may help you begin to discover that number.