China's cultural attitude is like a European power in the 19th Century.
Consider Franco-German tension over Alsace-Lorraine, or British reluctance to grant Ireland Home Rule, or the various Balkan Wars. These conflicts occurred over territories which the protagonists believed were historically theirs, and belonged to them inviolably, and it was good and just to go to war to keep them or take them.
China treats Tibet the same way. It could be a devoid, empty desert costing more than it brings in in resources and tax revenue, but that wouldn't matter - to China, it's a historical and integral part of the ancient Chinese Empire, and that is sufficient justification for them.