[thaumasiotes]

> But that's more of a geographic inevitability than some kind of fundamental cultural difference

No, speaking a different language is the most fundamental cultural difference there is.

> And if that matters so much, why does the Chinese government insist on holding on to these provinces whose culture is apparently unacceptably divergent from what they deem to be Chinese?

I asked this question of a Chinese high school student once. His response was that the Chinese didn't want to be conquered by the people of those regions. (Which notably happened in the 13th and 17th centuries.)

Wikipedia says that when the Qing dynasty fell, the idea was brought up that Xinjiang and similar regions should be divested from China as not being Chinese; they see the retention of non-Chinese territory as more a matter of no one being willing to take the responsibility/blame for the country getting smaller.