Exactly. They’re more than capable, physically, of being adopted at eight weeks, but many if not most puppies don’t make the emotional/psychological shift until nine weeks. I.e.: some (I don’t know the percentage) will grieve the loss of their family at eight weeks; at nine weeks, they are eager to leave the litter and bond with their new primary humans. I have see this with my own eyes. It’s horrible to watch a puppy get depressed over losing his mother and sibs. It happens at eight weeks, but never at nine.
We brought home our most recent pup at 7 weeks, but we have 3 adults already, so he seemed to feel right at home. He loves people and other dogs.
He came from a litter of 16, and wasn’t getting much attention from his mother, although she is a very sweet girl. That’s just a large litter.