Because Taiwan is an independent democracy
Really? So who is their representative to the UN?
Who is the US Ambassador to the Independent Democracy of Taiwan?
Oh, no, wait. They don't have one. Because the world already recognizes that there is only one China, and Taiwan is part of China.
Taiwan was fine with this from 1949 to 1971, when it was allowed to keep China's seat in the UN at US insistence.
The biggest problem with Western Policy Makers is that they say these lofty things about "independence" and "Democracy" when continuing the destructive policies that began with the first Opium War.
Taiwan is "independent" because the world let Japan steal it in 1895. Then, when Japan was forced to give it back, the US promoted the fiction that Taiwan was the legitimate government of China for 30 years.
China looks at issues like Tibet, Hong Kong, Taiwan, and the Uyghurs as examples of the West infringing on its sovereignty.