Read here what happens to puppies between 7 - 9 weeks of age in the development process, from the Doctors Foster and Smith website:
Puppies: What to Expect the First Year
Due to that delicate time, most don't want to stress a puppy of that tender age by forcing it to change its secure birth environment. You'll note that they say that at about 12 weeks, they regain their former puppy confidence and ready to begin learning simple things, have better bladder control and their sleep patterns are longer, often allowing for sleeping through the night.