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This blogger has a point, with mobility, resources, and something resembling an organic planetary INTERNET accessible at all times for hundreds of years why aren't they more technologically advanced?

An explanation could be they were highly advanced, but some doomsday weapon wiped everything out and turned back the clock in terms of technology. 

It's interesting to consider avatar from this perspective. Scientist have often wondered about different categories of civilisation. Perhaps they happen in stages. From a roaming family of creatures, to a tribe, to a government, nations, coalition etc.

The Navi would probably be a tribe stage species(not considered a civilisation yet). While it could be concluded that a tribe species is technologically inferior, it is also possible that they may have knowledge that even a civilisation may not have due to differences in the ecology or their planet.

Think about it. If the Navi could use guns, cannons etc, they would need an industry. A system to govern a larger population for mass production and heavy mining(which goes against their tribal philosophy in the story).

But most of the reason why the Navi are the way they are is, I suspect has more to do with it being a fictional story rather than a realistic look at what other intelligent life may be like.