Humans are a story-telling species. The way that you motivate millions (or a billion and a half) people to work together towards something is by weaving narratives. We humans invent abstract concepts like justice or money or states and religions, convince ourselves that the world revolves around them, and then move heaven and earth to make the world fit with our story.
One of the most impoetant stories to the entity we call China and the people who make it up, going back thousands of years, is the story of the Mandate of Heaven. The idea that China, long united, must divide; long divided, must unite. China goes through cycles. Noble leaders rise to unite the country; they become corrupted and lose the Mandate of Heaven, their nation falling into disrepair; they are conquered by outsiders or rebellion from within; a new Dynasty forms. This is the natural way of things, and how the world should be.
Now, Communism tells a very different story, one of class struggle between the owners of capital and the workers. And yet, China’s government has managed to merge the two narratives. The cyclical rise and fall of dynasties is over. The Communist Party is the natural conclusion to history.
But in order to prove that they are above the dynastic cycle, the Communist Party must show that they either possess the Mandate of Heaven, or are in fact completely above it. How do you do that? You unite all of China under your rule, forever.
If areas that were once China but no longer are get too far from the Chinese government’s sway, their legitimacy is called into question. Only by ruling undisputed over all of (greaterest historical extent) China does the CCP demonstrate this legitimacy.
In a way, I think it is similar to why ISIS propaganda claimed that they wanted to establish a Caliphate that encompassed all lands ever ruled by Muslims, from India to Spain. Anything less is an admission that you cannot legitimately claim that your authority derives from 10,000 years of Chinese history, or the Abbassid Caliphate, or whatever.
And hey, at the end of the day, that’s basically what Manifest Destiny was. God GAVE US this continent, and we prove that by taking ALL of it.