1. The Ethics Problem. Puppy mills are still around and still bad, and breeders in the past probably did some pretty terrible things to their dogs, but the human analog of the work performed by even the most humane dog breeders would still be a terrible travesty of the most basic human rights. Maybe I'm not creative enough or open-minded enough, but I can't conceive of a way to do this ethically.
2. The Nurture Option. A well-trained bulldog can be a much, much better dog than an untrained border collie. And one person can accomplish this training with a dog in a few years. Why spend 12,000 years breeding humans instead of putting a bit more effort into training the perfectly serviceable brains that we already have?
I tried to find some statistics to back up the latter point. The US spends about 2.7%, or 102 billion of the 3800 billion federal budget, on education. States average about 30% of their budgets, or (very roughly) 550 billion of the 1850 billion sum of state budgets, on education. A further complication is that much of education funding is done using local property taxes, which vary wildly by location, but are generally similar to the sum of federal and state funding. This is a total of about 1.2 trillion in education funding. Honestly, that compares pretty favorably to a 19 trillion GDP. And it ignores all the extremely important effort and money spent by parents to educate their children, and private spending on post-secondary education.