I, from what i can get from personal memories, is not that im having fun any less really. I mean i must have played through LoZ:Link to the Past at least 50 times if not more. Now it wasnt because the game was so awesome i had to enjoy it that many times over. It was because I didnt have any other games at the time or a job because i was 8, so i tricked myself into thinking the game was still fun just to have something to do. I believe the mentality as we get older is that games aren't as much fun, but really, the old games weren't any better, we are just lying to ourselves that older games are oh so awesome. Its just today we have more money to spend on them vs. having to wait for a birthday or holiday for a couple new games, therefore we blow through them faster and our time is spaced out over 10-20 games vs just 2-5.
Back when all I had was a Nes or Genesis and limited amount of games it was no problem to replay them, plus generally being shorter games. Now with having all current consoles, and still playing games that are current and bits from all other consoles over the years, coupled with many genres i try out these days.
Just way too much out there to enjoy to have time to replay. I still replay Kid Icarus every so many months but that only takes an hour. Shmups, light gun, racers are fun and short enough to replay throughout, but long campaigns and most games I'm satisfied with one go. --- "you will find you spend a good deal of your life sitting at red lights"
I watch movies more than once. The few books that I read, I may read more than once. I play my games more than once because well, they are fun, and I usually gather more about the story and backstory, which I may have missed the first time around.
I actually replay a lot of my old games. It's funny because even if a game has replay value, I tend to play it the same way I always do. For example, I've beaten every Silent Hill game several times but never gone after the UFO endings.
People ask me why I don't sell some of my 300+ game collection. Well, I intend to play them again in the future.