Yeah, what the heck is up with that? How is it that China claimed something that was a dispute between India and Pakistan, and nobody really cares?
Is it basically uninhabited but useful as a defensive boundary for China?
Chinese Kashmir was the part of Kashmir the Pakistanis had taken right before the Maharaja signed the Instrument of Accession to join India back in '47. The Pakistanis awarded that piece of Kashmir to the Chinese later, as a gesture of friendship. India doesn't care because they don't want to take on China yet. They need to fry the smaller fish in their neighborhood first.
Because they could... and nobody (besides India) cared.
So, they did. China annexed the Aksai Chin, because they could get away with it, and they knew it.
Is it basically uninhabited but useful as a defensive boundary for China?
Defensive?
I would not regard China's seizure of the Himalayan high-ground overlooking Indian Kashmir as necessarily being defensive in character.
CHINA
INDIA