Recovery of lost territories and reunification of the country is a core policy of the Chinese government(s) since 1912...
And as noted in the replies, striving for national unity is an ancient theme of Chinese national psyche.
Stamford Raffles "took" Singapore from the Malay sultanate in the 1820s. In 1963, we finally got it back, but let them go barely two years later due to ingrained cultural differences that manifested during the meantime.
With the rise of so many capable cities on the mainland, perhaps pursuit of unification is just not worth the trouble anymore.
Maybe to you? It's clear from China's actions that they think otherwise and they're willing to do whatever it takes to ensure control and unification.
This is very different from HK and China.
There is their whole mythology of "centuries of humiliation" and their warlord period for actual stand together or hang apart sentiment. However the sort of "unity" talked about is really just propagandized expansionism. I see it no different than Putin's foolish revanchism for the former USSR.
I wonder how much of that unity sentiment is/was constructed and pushed on by leadership due to the area around current China being littered with competing kingdoms and a lot of internal strife and disarray throughout its history, not just the past 100 years.
In Chinese cinema, or at least some films, the theme of unity and to some extent 'glorious leadership' seems to be prevalent. Just came to mind that even in some Jet Li films (or equivalent) set in more ancient times you could probably be faced with some monologues or statements to that tune and at least I was a bit taken aback by how blatant it felt.
It's trying to push a narrative to argue what is part of the country...
Almost everything we appear to hold so dear and immutable or self evident seems kinda arbitrary depending on how far back you go and whether you consider yourself part of this team or that.
Nationalism or tribalism is a whole lot of make believe, is what I'm saying.
7 and a half centuries before Yue Fei, there were Liu Yu[1]. 100 years before him there was Zu Ti. They both tried to fight northern nomads, and were remembered till this day due to their success.
Unification of the nation IS in the genes of the people. There is no question of that.
[0] https://en.wikipedia.org/wiki/Yue_Fei [1] https://en.wikipedia.org/wiki/Emperor_Wu_of_Liu_Song
Citation needed.
Cherry picked facts are not universal truth...