There's a kernal of truth here, but also a manufactored truth that's there because the author wants to see it.
Whites write about whites because we're whites. If we write about blacks then Spike Lee tells us we've got it all wrong, or we're accused of cultural appropriation, or whatever. We're white, we write about whites and there's nothing wrong with that.
This leaves you with two basic storylines, as far as alien or foreign cultures go: White goes foreign, or foreign comes to white. At which point the white culture is either friendly and accepting or it's aggressive, challenging and overwhelming.
The friendly co-existance doesn't lend itself to much drama, it doesn't challenge our self perception. So we're left with primarily story of cultural clash.
Whites aren't always the bad guys. In Conrad's Heart of Darkness, the white culture is safe, structured and civil, and the alien, foreign world is scary and dangerous and borders on evil. Many stories came from that throughout the 20th century, to the point that it got pretty predicatable. Storytelling broke out in the 60's, primarily with growing questioning over Viet Nam which challenged our indoctrination that walking in and "civilizing" a nation, putting a layer of our culture over top of whatever we found, wasn't necessarily beneficial, much less ethical.
It's a common theme because through the 19th and first half of the 20th century there was a lot of indoctrination within storytelling that promoted the idea of white cultural and ethical supremacy and the benefits of colonization and paternalism. Take a good look at Narnia
for an example.
Whites aren't any better or worse in their behaviour. Native American tribes committed genocide and practiced slavery before whites set foot here. The Japanese committed horrible atrocities on Chinese, and the Chinese committed the same on other neighbours. Blacks were captured and sold in Africa by blacks to whites for shipment to America. People aren't nice. It's not about colour.
But what it is about is that whites are writing stories for whites and some of those stories are intended to challenge our perceptions about ourselves. I think Avatar
a bit childish and obvious in it's theme and story, it's just a tale that has been told elsewhere in a more compelling way. It's overly simplistic. The natives are "nice", they don't rape each other, or commit atrocities or damage the environment and the walk away to another plot of land. The way Native Amercans did before the white man came. We don't see that, what we see is clear good/bad dichotomy. So it's simple storytelling, a simple message.
What should be the compelling part of such a story is the journey of the protagonist away from his world into the foreign world, the transformation of his outlook and personality, so that the indoctrination we all have from our culture is shattered. It's not about (or shouldn't be about) our culture being "bad", it's about culture itself being bad, about it limiting us, controlling us, shaping us, imprisoning us. The journey of the protagonist in Avatar, Dances with Wolves, Last Samurai, etc, should be about breaking the mold.
It's presented to us in the format we usually see, the format of military, of clashing of cultures, because for many the indoctrination of love of military and love of country and love of our culture and society and values and belief that we are the "best" is not only so strong, it is so dangerous. It is practicly a fetish, and witness the popularity and outright drooling over the military in popular shows like Battlestar Galactica. The idea is to challenge and break this mold.
The question shouldn't be "when are white people going to stop telling stories like this" but when will humanity finally break away completely from the chains of all forms of cultural indoctrination. When that happens, we won't see stories like Avatar because they will just be quaint relics of past generations. Maybe in another couple thousand years.