Hmmm... Since the Na'vi can connect to their supercomputer and access all their ancestors knowledge, they don't need to advance so much.

I mean for transport they have the horses(not sure what they are called) and ikran/banshee for flight(though only for one person).

And also since advancement needs sacrifices (cutting trees,minig for minerals and oil), the Na'vi can't do that as they are linked to every other living thing via their supercomputer.

The strange thing is how come they have carbon fibre in their bones when the gravity their is lesser?