There's a lot of people who ask questions about history or geopolitics that amount to, "why would <country> pursue <goal>?" when <goal> is a central ideologically motivating factor for <faction-that-controls-country>. The classic example being Germany's 1941 invasion of Russia--often derided as a strategic blunder in hindsight, except the rulers of Germany in 1941 had the conquest of Russia as their primary geopolitical goal in the first place. Likewise, the existence of Taiwan outside of CCP control is contrary to the goal of unifying all of China under CCP control.