The majority of the people of Taiwan are ethnically Chinese, but this is a relatively recent status. Taiwan is not an ancient part of China.
Taiwan has become ethnically Chinese in 2 stages, first an immigration from the neighboring Chinese province that is a few centuries old, then the invasion of the island by Kuomintang at the end of WWII, which took the political power from the native Chinese.
So Taiwan has become a Chinese-populated territory only during the last few centuries, and the desire to unite it with mainland China is not something that can reassure China's neighbors that this is where its desire of expansion will stop.
Taiwan has become ethnically Chinese in 2 stages, first an immigration from the neighboring Chinese province that is a few centuries old, then the invasion of the island by Kuomintang at the end of WWII, which took the political power from the native Chinese.
So Taiwan has become a Chinese-populated territory only during the last few centuries, and the desire to unite it with mainland China is not something that can reassure China's neighbors that this is where its desire of expansion will stop.