Most of the answers to this question seem to assume some type of material or geopolitical calculus. The issue is far more cultural.
Begin with the simple observations that reuinfication is popular with mainland Chinese people and party members and that the official position of the PRC since its inception has been that the PRC and ROC are one country. Taiwan's independence is an affront to the perceived authority of the mainland government. Reunifying China after its fracturing under imperialism is a deep cultural ethos. Material concerns are secondary to this.
Begin with the simple observations that reuinfication is popular with mainland Chinese people and party members and that the official position of the PRC since its inception has been that the PRC and ROC are one country. Taiwan's independence is an affront to the perceived authority of the mainland government. Reunifying China after its fracturing under imperialism is a deep cultural ethos. Material concerns are secondary to this.