Those places have their own culture, identity, political views, and way of life. All of that gets erased and replaced with the CCP’s ideology and Han Chinese culture through a process called Sinicization (https://en.wikipedia.org/wiki/Sinicization_of_Tibet). It’s a very effective way of erasing and controlling people.
Plus given that the government of China is very authoritarian and controlling and not democratic, I don’t think it’s good for the world when they get more land, resources, and economic power (which turns into military power).