The floating rocks contain Obtainium.. totally different kind of material

You mean the relatively small deposits that are significantly more difficult to mine?

Damn, there goes the carefully crafted realism of the blue space cat Ferngully series.

Richard Dean Anderson? I think he's more of a C4 kind of guy.

Strip mining is always easier for the mining company. Also it sets up conflict between the humans and Navi.

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.

Too high.

This video actually provides a great insight into that question: https://youtu.be/4Ugebzq3juE

This may be the best answer.....