It doesn't matter how brilliant and classic they are.
This movie for instance has some of the most hilarious and iconic scenes with arguably the most revered comedian of all time.
Yet it's average rating is only 7. The other Pink Panther movies are between 6 and 7.
Why is that?
Combination of factors...
1) can't spot irony or contradiction in their own social circles;
2) don't regularly seek out very diverse comedy material from different eras;
3) in person they just aren't that funny.
Comedy is one of the most subjective things in entertainment. Reactions and tastes always vary.
shareComedies don't always age well. Something innovative and cutting edge becomes old hat. For example, the campfire scene in Blazing Saddles was funny because that kind of humor wasn't done all that often in the 70's. Today, fart jokes are a dime a dozen. Someone watching Blazing Saddles for the first time after growing up with such jokes, wouldn't find them funny.
Also, I think we remember a movie being funny on rewatch more than actually finding it funny, anymore. We don't laugh the way we once did because we know the jokes and when they're coming, but we recall how we felt when seeing them for the first time.
It's why having interesting characters and plot are so important. They still keep the movie enjoyable once the gags have lost their luster.