That isn't strictly true. New neurons do get created in adult humans, in at least two regions of the brain, but probably more. There is even some evidence of cannabis potentially contributing to this process.
That very Wikipedia article says that adult neurogenesis essentially does not exist in humans, according to the latest studies.
This used to be a very controversial topic, and some people still hold on to the older view, but new research has found nothing in the last place where it could have been plausible.
https://en.wikipedia.org/wiki/Adult_neurogenesis
EDIT: typo