First, thirty years have passed since then. Marty was in 1955 for only a week, and in that week he was actually with George and/or Lorraine for a matter of hours. Yes, he had a big influence on their lives, but all the same they did not see him very much, and it's not likely that they would remember his face after thirty years. Many of us have trouble seeing the faces of people we've seen hours ago. Second, it's not as if they suddenly see Marty again out of nowhere. He is their son; they had brought him up from birth, seeing him grow up through the ages. Even if they did think there was a resemblance, wouldn't it be likely that George and Lorraine would either a) put it down to a coincidence, e.g. Lorraine: "Hey George, you know who Marty reminds me of? That guy Calvin Klein we knew for a week in High School!" George: "Yeah, I remember him. Ha! That's kind of funny since that's where we came up with the name Marty, huh?" OR b) consider it to be the same guy? Lorraine: "George, I can't believe this, but I think our son Marty is the same guy we knew for a week in 1955!" Of course, it not mentioned whether George ever noticed the suspiciously close resemblance between the boy he thinks is his son and his wife's old boyfriend, or if he ever broached the subject with Lorraine. They don't even remember Marty being the one who got them together. At the end of the movie, they mention that if it hadn't been for Biff, then they wouldn't ever have gotten together. So, why would they remember him?