MG was released 29 years ago. So you can say about the parasites, etc; They get to retcon, changes a lot of things in every new game - probably its a tradition and 'it's ok'. «...back in 2011 how MGSV would be structured. At that point, all the elements were already in place; the themes of phantom pain, the vocal cord parasites, race, and retaliation, and of course the Moby-Dick motif.»
Also: «Kojima explained that we’ve come a long way since these games came out, and looking back, a lot of things in the plot don’t make sense anymore. He would also have to change a lot of things to bring it up to todays standards. If he had to redo the original Metal Gear games, he feels that he would have to rewrite their entire stories. And that’s something he’d rather not have to do.»