The Human Brain in Numbers: A Linearly Scaled-Up Primate Brain

And even then, along the way you might accidentally breed out some other important gene. Humans are also not machines, and require a great deal of emotional care to develop into functioning, centered adults.

There is probably a trade-off between generation time and brain size. Human babies already require substantially long care even by mammalian standards, because the brain cannot develop fully in the womb. We also take an extraordinary amount of time just to be able to walk on our own. Most animals are born with the ability to stand and move around.

Perhaps genetic engineering technologies such as Crispr will make it practical in the not too distant future.