But it doesn't do that instantly and it does it for good reason, eyes and the sections of the brain using them require energy and are vulnerable to infection so in situations where they don't provide an advantage they increase the likelihood of death before breeding thus giving any offspring born with less energy devoted to eyes has a small advantage which over s very long time results in them being selected away.

So unless the creatures reach a perfect form for their environment then they'll always be in the process of changing and have some of the old junk in there. Also if the formerly useful part doesn't make any real difference to survivability there's no force driving it to be selected away from, it might eventually be removed by lots of pure chance events but that's going to take a huge amount of generations meaning the middle time where there's junk not yet removed us going to be very long