it's like Spain wanting Catalonia to be under control. Why would Spain want to lose control of a part of their own country?

Taiwan is a single point of failure for chip manufacturing in the world I don't think annexation would destroy all the chip fabs I think they would still exist and they would be Chinese

china is a bit like Yugoslavia before the end: lots of different ethnicities being forced to be together. letting a country made up of Chinese people exist in parallel to china keeps a flame of hope alight for those 51 non-Han Chinese ethnicities that were forced to be part of continental china. and China has struggled immensely with multitudes of local kingdoms and warlords throughout its history so it is afraid as its people are very aware of this past through historical dramas

Extreme disdain for democracy. Some psychopaths want to bring us back before the age of enlightement, while using enlightement or (especially in case of China) post-enlightement ideas to do so, because we citizens are all like badly behaving 12 year olds, who need 24 hour supervision on every day on the week, every month of the year, or we might "go insane" from doing something a psychopathic madman does not want us.

Money, power, influence, prestige

Evil, Scary China Refuses To Passively Let Us Encircle It: Notes From The Edge Of The Narrative Matrix – Caitlin Johnstone

That's a pretty good article explaining it. The funny thing is that the US media is always framing China as the aggressor. But one look at that map, like with your real eyes, not the crazy eyes, should show you the US is way out there on someone else's doorstep and who the aggressor is. That's just geography.

Personally I don't think China is going to invade Taiwan unless things escalate further. For example, the Russian perspective on the Ukraine is that the US supported the regime change through the NED, helped far right elements overthrow the democratically elected regime and then supported their stance to ban Russian language, oppress Russian speaking populations in Ukraine and supplied them with massive amounts of arms and intelligence. All of this is true historical fact. And in that situation even the chief of NATO Stoltenberg publicly said that Russia launched a "preemptive war" in response to this quasi-NATO membership right on their doorstep. If the US does the same with Taiwan, China might invade. That particular gabit is rather unlikely to succeed in Taiwan though, and Taiwan is far less dangerous to China than a hostile well supplied Ukraine is to Russia (only like 500 miles from Moskow). The smart play for China if that happens is to play rope-a-dope until the US gets tired. Kinda what Iran is doing about the numerous provocations and acts of war against them.

Street cred.

China considers taiwan a part of its country. If china is able to capture and integrate taiwan into it then it will show that china has become a superpower. If china can capture taiwan then it will show USA and west that china is not a paper tiger.

The premise of your question is all wrong. It centers western control of Taiwan as a natural status quo, and so paints every challenge of that control as a provocation or threat.

Advocating for Taiwan’s sovereignty is not the same as advocating for Taiwan’s continued fealty to the west.