Because the china we know today didnt actually exist until 1949, despite the confusion. The reason why you see taiwan is because the republic of china government was established in 1912, but before moving into the island, they had the mainland. They were then under a civil war until 1949 when the communists, now known as the peoples republic of china which is the one we know today, took over the mainland and then the republic of china relocated to the island, and a bunch of countries severed their diplomatic relations to recognize the PRC as the real china, which i totally oppose but this is just history.
lol
*confusion*