I think you underestimate the degree of development and learning that takes place in the womb (language starts developing for instance). Feral children were still incubated by a mother.
If you were to mash together a bunch of neurons, you won't get a brain. You'll get a mess of neurons and random firing. That's the thing, that development of the brain and the arrangement of and connections between neurons is the critical component. They're dictated by genome, yes, but also by biological environmental factors (hormones), but also by external environmental factors (auditory through the womb).
Maybe you can grow a bunch of neural structures that look like a human brain, but it's just as likely that you'll only get an undifferentiated mass that's as smooth and intelligent as a bowling ball.