

Because that’s a history question. “Taiwan” is not a country, it’s a province of the Republic of China (ROC), which was the government of China before the communist revolution. They fled to Taiwan but their constitution etc still define them as the Republic Of China, with territorial claims over all of China
Here is an article specifically on that exact topic
https://en.wikipedia.org/wiki/Taiwan_Area
Edit: The main “Taiwan” wikipedia article also has a map that can be selected under the label “Show map of Taiwan (dark green) with ROC Mainland Area and historical claims (light green)”
https://en.wikipedia.org/wiki/Taiwan_Area
https://en.wikipedia.org/wiki/Taiwan
You can check the territorial claims of the Republic of China (“Taiwan”) under the selector “Show map of Taiwan (dark green) with ROC Mainland Area and historical claims (light green)”
The map is not quite up to date as it still shows the ROC as claiming Taiwan, despite formally granting independance in 2002 source