Well they don't all say that, some report no experience at all, others tell of simply arriving in a heavenly (or a hellish) place.
But the sight that the infant has while passing through the birth canal would not match up with the "tunnel" story. Its eyes would be pressed against the side of the birth canal and it could see nothing at all until it had come out as far as its eyes, then as far as it knows it is completely in the light.
How do you know that when a baby opens his eyes, he immediately sees light?