Ego.

China wants to be world hegemon by 2049. That's no secret. They've been very clear about it. They picked 2049 because that will be the 100th anniversary of the CCP's rule of China. Everything China does is aimed at that.

And what kind of hegemon would China be if it allowed Taiwan to defy its will. China's ego is too fragile to tolerate that.

Xi Jinping wants to be the one what got Taiwan (just as Putin wanted to be the one what got Ukraine). He is 69 years old. As he gets older he will agitate to get it done. He will lose patience with the long term effort at destabilization of Taiwan and he will go to war.