Because it is where the US will launch a false flag attack to justify invasion of the mainland, they need to remind their peons that it actually exists before they do so.
Taiwan houses TSMC who are the best chip manufacturers in the world. China is behind on that and is signaling intent to take over Taiwan by seemingly gearing up for war while talking about historical claims.
Historical claims? Lol wut? There is no country of "Taiwan" they call themselves the Republic of China and are remnants of the chinese civil war, specifically the nationalist side who lost to the communists who control the People's Republic of China. Without the amerikkkans sending their fleet to prop up and protect the ROC government in Taipei there would be no division and Beijing would have achieved Chinese reunification long ago.
The PRCs claims on the island of Taiwan are legal and recognised by the most every country on earth including the USA since 1973 via the upholding of the One China Principle.
Out of all of American anti-china pro-war talking points, this is the most insidious one that I have heard floating around.
The UNITED STATES is the one that sees TSMC as their core interest in Taiwan, hence they created immense pressure on TSMC to open a branch in Arizona, promising all kinds of benefits, but once there they revealed their actual policies will practically force TSMC to provide its trade secrets to the US government.
To the Chinese people, Taiwan is a lost piece of land and a lost group of people that is separated by a civil war where the mainland has long since won by sheer effort of being bigger and basically the recognized the government of China. There are strong emotions, poems, songs dreaming about reunification, for decades (from both mainland side AND Taiwanese side, btw). Whether one accepts the validity of their perspective or not is fair, but the fact is those desires are based on wanting to right a historical wrong, a period of humiliation where the country was broken apart, and wanting a lost relative back into the family fold.
The idea that that China wants to reunite for ECONOMIC reasons is sheer absurdity. And viscerally infuriating. A disgusting demonization, honestly. As though a China that can put a space station up into space by itself, after being barred by Americans from participating in any American based space initiatives, will have trouble to produce chips of any size in a couple of years now that there is the need to do so.
Chinese chip tech and production capacity will blow any western options far out of the water in a decades time (probably less), if literally every other industry on earth is any indication.
Chip blockades are a short term annoyance for China and nothing more.