Taiwan is the closest part of the first island chain. The first island chain consists of Japanese, Taiwanese, and Filipino islands that are all western-aligned and could be used to blockade the entire Chinese coast. China sees this as a strategic vulnerability. Taiwan also used to be part of China. Combine all that with the "century of humiliation" narrative and you create the political conditions for an unwavering determination to take Taiwan as a way of reclaiming their spot on top. I doubt it has anything to do with industry, economy, or resources. It's just political narrative. Taiwan could be a barren desert. As long as their flag looks different and they're cozy with the west, China would still want to conquer them.

Chip fabs. TSMC is leaps and bounds ahead of Chinese production.

Probably a bunch of other tech stuff too, but that's the big one.

Because that's how fascists work. China's built up this image of a victim of these great crimes, and that they must work to bring justice to them. One of those is the perceived loss of territory.

To stop at least pretending to work towards that would be a large cultural disillusionment moment and challenge to their core identity.

It's not only about Taiwan itself and their tech business there as others already have said, but about the entire South China Sea - and the sea’s estimated 11 billion barrels of untapped oil and 190 trillion cubic feet of natural gas. China competes with other claimants in the region such as Taiwan, the Philippines, Brunei, Indonesia, Malaysia, and Vietnam.

China has been steadily increasing its assertiveness in the South China Sea since the 1970s, resulting in heightened tensions with Southeast Asian states, particularly the Philippines, at the Second Thomas Shoal in the Spratly Islands, which possesses rich natural resources and fishing areas.

In 2016, the Permanent Court of Arbitration at The Hague ruled against China regarding the Spratly Islands after a claim brought to the court by the Philippines on the basis of the UN Convention of the Law of the Sea (UNCLOS). Although China is a signatory to the treaty establishing the The Hague tribunal, Beijing refuses to accept the court’s authority to this day.

Over the years, China has even increased its efforts to claim land in the South China Sea by physically increasing the islands’ size and even creating new ones altogether, e.g., by piling sand onto existing reefs. In addition, China constructed ports, military posts, in Spratly Islands and the surrounding area. China has also been deploying military jets, cruise missiles, and a radar system.

This is about money and colonial power at a much larger scale than "only" Taiwan and semiconductors.