The Avatar movies serve two purposes for James Cameron: world-building and technological boundary pushing.
World-building: James Cameron loves imagining the ecosystem of Pandora. All the different plants and animals that we may only see for a few seconds are each things that he has put a lot of thought into. It is hard to deny that, while the stories he has told in Pandora are not super exciting, the world itself has an impressive level of complexity.
Technological boundary pushing: The first Avatar film was basically the resurgence of 3D in movie theaters. It is the reason why every movie for the next 5 years was 3D. It was why people started buying 3D TVs. It could be argued that it’s the reason that the Nintendo 3DS was announced the very next year. Avatar took 3D from being some gimmick where things come out of the screen to a truly immersive movie-going experience.
The second film is all about the mechanics and challenges of making CG water look real and beautiful. Water has always been a hard spot in visual effects, but Weta patented a brand new technology for Avatar 2 to create “thin film interfaces” for areas where water is seen on the surface of textured materials. Another new technology was created for the process of filming motion capture underwater rather than putting actors on wires and trying to fake what moving underwater looks like.
While most viewers are not keen enough to know that all of these things are happening, our brains are very good at determining when things look fake. So, when it does look real, you notice even if you don’t realize that you notice.
Yes, the plot is a little lackluster, but that’s not why people love the movies. It is because they are visual masterpieces