I don't believe the lack of adult brain development has anything to do with genetics or evolution. I think it's entirely environmental; that society forces adults to accept intellectual stasis. Even if it were biological, some drugs have already been proven to restart neuroplasticity, even reverse alzheimers. But those who use them soon discover how severe is societal enforcement of intellectual stasis.
Uncle sam and the united nations kill or otherwise neutralize folks who take brain chemistry far enough to reveal the possibility of lifelong learning. The most promising central nervous system medicines are being commercially developed in europe since such research has been banned in amerika. Uncle sam's official position explicitly mentions sending the right message to our children.
Inner space and the blood-brain barrier are politically subjugated by force of law. With our political regime's stranglehold on focusing our psychobiological attention in ways that prolong the regime, industry has dedicated itself to erectile dysfunction and deadly me-too drugs like vioxx.
I'm sorry i personally have such a hard time shutting up about this sort of thing. I see the next phase of social evolution being thwarted by the fear and greed of a small but sinister elite.
Even things as simple as fish oil supplements and simple commitment to personal growth have clinically proven more helpful than the mercantilist state wants advertized.
Cultivating stupidity is both a profit center and a security measure. But that doesn't make it natural or optimal.