Why? Because you don't turn your back on your friends in their time of need, that's why. The Taiwanese have been independent from mainland China for hundreds of years. Jimmy Carter stabbed Taiwan in the back when he broke diplomatic relations with the Taiwanese in the 70s and they despise him for it. (Well deserved, IMO)
A truism regarding human behavior: if no contraints are placed on behavior, the individual or group will continue to push the bounds of behavior until something forces them to stop. The Chinese are currently seeing how much they can get away with. So far, resistance has been from non-existence to light. Expect their hegemonistic practices to continue until somebody (like US) forces them to stop.
How are they such good friends?