For a lot of us, we were kids in those times we look back at those old games, and as kids, we had so little other things going on that we could focus our determination on finishing those games. Now other items take priority, like work and family.
Plus most kids I knew growing up only had a few games, since you had to wait for birthdays or other holidays to get those games you really wanted. And since our attention was divided among only a few games, we'd often pick our favorite and play it over and over. Now I can just go out and buy a new game whenever I want, cutting down the replay time of old games.