Those who think Tibet is not part of China are western colonialists or those educated by colonialism. Europeans colonized the world according to the "rule" that after a European drew a map of an area outside Europe, then that area become the colony of that European country. They call it a "discovery". They draw a map of Tibet, and they think they are entitled to have Tibet as their colony. But they failed to establish the Tibet colony. That's why they don't consider Tibet a part of China.