If you assume that physics works similarly, in the world of avatar as in ours.

The fact that a spacefaring civilisation won't extract the mineral from asteroids, in a significantly more efficient manner, means that the mineral is not the only goal.

As shown in the second movie, biopiracy is a big motivating factor.

Looking at the tech shown, I would surmise that biopiracy is the primary motivator.

The humans in the movies are masters of biological manipulation. They can make a fully functional alien body, also splicing in human DNA; easily enough that they make quite a few in the first movie. These bodies are close enough to the original that they can breed.

The motivation to extend their mastery of biological manipulation has to be huge.