The reason for that is because both Taiwan and China insist that you either recognize Taiwan xor you recognize China, but you cannot recognize both. (This is basically what the One China Policy means).

Additionally, world wars are never triggered intentionally. World War I broke out essentially because the decision makers ignored the wider consequences of +1'ing their opponent's military preparations (one author characterizes it as Europe sleepwalking into war). World War II broke out unexpectedly because Hitler underestimated the spine the Allies had when he requested they add Danzig to his list of silver platters. World War III will likely break out when someone crosses a red line to one party that isn't clearly communicated to another party--and a Chinese attempt to invade Taiwan is one of the likelier options on that list.