IIRC, the missing stretch of the Pan American Highway in the Darian Gap has not been completed because of the illegal migration that would occur into Central America and beyond.
That could be a reason, but the Weekly Reader articles I was referring to were written well over a half century ago—before mass illegal migration became a thing. I think the hostile geography plays an even bigger role. That and the Darien Gap has some ‘interesting’ people living/doing business there.