The reason why Avatar hasn’t had the same cultural impact is because it’s from 10 years ago and has no follow up material. Marvel movies come out like every couple of months and has lots of additional material. You’re talking 1 film compared to a franchise universe with an entire corporate machine constantly backing it.
Gone with the wind
12 angry Men
Need I go on? Plenty of films have had much more cultural impact without any sequels.
Literally all of these are adaptations of written works, with many versions across TV, theaters etc. often before becoming movies.