Do we really know he was offered the opportunity to leave?
One version I’ve read is that all the pilots were given the promise of early release in exchange for signing statements critical of the US, and McCain, like the others, refused.
Another version is that he was offered early release, but he knew that being an Admiral’s son it’d be seen as preferential treatment, and he refused so as not to hurt the morale of other POWs.
What I can’t figure out is that if the North Vietnamese wanted to release any prisoner, how the prisoner’s feelings in the matter would count for much. I assume any enemy willing to torture would have no problem shoving you out the door against your will.