Avatar was from 2009. Children of Men from 2006. That just means those movies were big things back in the day, they're not anymore.
I can tell, for example, that 10 years from now, half of the movies from the last 5 years that are now in the top 250 will be gone. The two Avengers, for example, they're not bad movies, but they're not in the top 250 of movies ever by any means.
The Exorcist, The Big Sleep, The Grapes of Wrath, The Wizard of Oz and Touch of Evil are all movies that came out decades before and were well into the list without being threatened but have all fallen off. And it's not like they were bumped off because more movies came in, their scores got lower.