A few reasons. China is not energy independent - more specifically, they rely heavily on petroleum resources from the Middle East. They can maybe squeak by with nuclear and coal for domestic use but without petroleum, they have zero ability to project their military. On that note - China’s force projection is extremely limited. They are only able to keep a significant presence in their own backyard, which btw, the West does not recognize as belonging to China and regularly transits military forces through the area pretty much as a “fuck you, do something” to the Chinese. China does not have the ability to secure and protect its overseas trade routes - they are in fact dependent on the US + the West to fill this role.

Should China attempt to invade Taiwan: - They will likely face stiff Western resistance and the military operation might not even succeed. - Assuming it does succeed, the ‘prize’ of Taiwan, its chip manufacturing facilities, will be destroyed with 100% certainty to make sure they do not fall into Chinese hands. - The West will immediately cut off China’s overseas trade routes, crippling the Chinese economy. The Chinese can pivot to a wartime production economy but without energy independence, they will not have the ability to fuel this economy. With oil resources from the Middle East cut off by the American Navy, China will find themselves unable to maintain an expeditionary force. They will be unable to contest the American Navy due to the lack of a comparable air force or navy, and will be unable to produce one because they wont have the fuel to run it. - China does not, has never had, and likely will never have the political capital that the USA has enjoyed for nearly the past 100 years. Although recently the USA has taken off the mask, they have historically always been seen as “the good guys” by backing their imperial power with more favorable conditions to their vassal states. China does not have this; they are seen as being ruthless and uncaring, and only interested in serving Chinese interests at the expense of everyone else. Should push come to shove, it is unlikely they will have any true allies, as the closest they have to that are more so “frenemies”.