The human brain is not a single organ, but two separate systems that evolved independently before fusing together, according to groundbreaking new research. … Armed with the knowledge of how hindbrain cells uniquely form, the team has successfully turned human stem cells into becoming functional hindbrain motor neurons in a lab for the first time.
It’s that the brain stem is made of different cells than the forebrain and midbrain. You can’t grow a full brain from one sample of cells. So, it’s considered 2 different organs.