Because then all your plans have gone to shit. If you're going out, you have to re-shower, re-dress, etc. If you were going to work on something, you're groggy. If you're going to cook something, you can't be bothered to get dressed again to go to the store. If you had planned on going to the gym, all the good machines are inevitably taken by the time you wake up.
I feel sleepy.