[CountSessine]

Certainty.

The CCP has (right or wrong) declared that Taiwan is an inviolable part of China and a rogue province. That means that if Taiwan were to declare independence and succeed, it would be a signal failure for the CCP - a gross loss of face. The party's legitimacy (always a complicated thing for an authoritarian state) would be threatened by the demonstration of weakness.

Taiwanese independence is the CCP's soft and fleshy underbelly. Bringing Taiwan under Beijing's control eliminates the uncertainty that comes from that political vulnerability.