I did a bit of research, and listened and read a few James Cameron Interviews. There are a few reasons. It is the largest scale pre-production in history, and there are good reasons:
1. He isn't making one film, it's four.
2. They fleshed out the Polyphemus system complete with planets, ecologies, and histories. This began during the making of the first film and continued for years after. Not only new planets and moons: Pandora has also been expanded.
3. They wrote an epic scale story, it took four years. BONUS: Part of the plan is to release the four novels alongside the films. While they had the bullet points before, the full development took from 2013 to 2017.
4. WETA required many upgrades and additions which included new types of motion capture studios required for the production.
5. It's James Cameron, perhaps the most meticulous auteur in the film industry.
That's why it has taken this long.
That was pre-production. As for the production timeline, it's long too, almost a decade;
Early 2016 - Production kickoff. Assuming this is when the major work on set construction began.
Early 2017 - WETA (of Lord of the Rings fame) began work.
Summer 2017 - Filming for Avatar 2 and 3 began.
2019??? - Marketing push for Avatar 2 to begin.
2020 - Avatar 2
2021 - Avatar 3
2024 - Avatar 4
2025 - Avatar 5
In total, the project will be 15 years and will 1 billion US dollars, the most expensive in history.
Will the project be successful? Who knows? Keep in mind Titanic was not expected to be anywhere near successful given its genre and length. Even the production company (Fox) had no faith in a 3-hour historical drama. As budget projections ramped up to 200 million USD (then unprecedented), they made demands to cut the film - they argued a shorter sub 2-hour length would be both cheaper and may attract more repeat viewings. Yet Cameron defied them. First, he offered up his full share of the earnings; they rejected this. He took a more drastic approach and held the film ransom, "you'll have to kill me." were his words. In the end, he got his way. One of the few auteurs that successfully put his unchecked ambition to the screen. It paid off, over twenty years since its release, Titanic remains unconquered at the box office... With the one exception of Avatar.
The group of sequels is on route to be the most ambitious film project taken to fruition in film history. James Cameron's magnum opus.