Why do some people think Tibet is not part of China?

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.