Even people "born into" a religion are responsible for their own decisions once they become legal adults. So anyone that gets baptized after reaching the legal age of adulthood has chosen this religion.
Why do others convert?
Nobody goes looking for a cult, cults go looking for them.
JWs come off as good, honest "nice" people. They honey trap people then bait and switch them.
With JWs the religion you join is not the religion you find yourself in. This religion has a public face and an inner reality that is very different. If anything, born ins should be more aware of this disparity than converts.
Being recruited by a cult has nothing to do with intelligence Steven Hassan explains this very well in his book.