The deliberate sending of a US Air Force B-52 bomber on a fly-by mission over a Chinese-controlled man-made island in the West Philippine Sea was a calculated risk taken by the United States to test both its claim to freedom of navigation and air space over international waters and China's unilateral claim of sovereignty over the entire area.
The fly-by, which followed a similar one by a spy plane with journalists on board and a couple of sail-throughs by US Navy ships, including the most recent by a guided-missile destroyer, expectedly provoked very angry reactions by China. That China has never taken direct action in all of these "provocations" other than angry reactions can mean any or a combination of several factors.
One is that China may not have been sufficiently provoked or angered enough to take direct action. While clearly nettled by the US actions, China has not really lost any significant thing other than, maybe, some face. But losing face is the least of China's problems. In fact, if face truly mattered to China, it would not have embarked on a territory-grabbing spree in the West Philippine Sea in the first place.
China knows it does not own the entire West Philippine Sea. The whole area is broken up into several other territorial claims involving a number of countries like the Philippines, Vietnam, Malaysia, Brunei and Taiwan.
The only thing China owns is the gumption to push its weight around and the knowledge that the United States, weakened by involvement in many other conflict areas in the world, may not be as quick and resolute in responding to Chinese aggression.
Not that it is the fault of China. The US itself has been advertising its weakness. It no longer barges into any conflict situation unilaterally like the global policeman that it used to be. Now it has to summon its allies to join it in a coalition for any and all of its military endeavors. Even in the West Philippine Sea, the US would probably not have sent probing missions into the area had both Japan and Australia not shown significant hints they agree completely with those missions.
Another thing that may have held China back against taking direct action is that it is not stupid. It is fully aware of where it stands. It fully understands the reality of the situation. China knows that its claim to the entire West Philippine Sea is not backed by a single historical fact. The only thing impelling its claim forward is its ill-disguised need for food and natural resources that the West Philippine Sea contains and can provide.
The West Philippine Sea has vast marine resources to feed China's 1.3 billion and still growing population. It also hosts untold mineral deposits that can continue to power China's great economic surge forward. Such pressing and drastic needs are more than enough for China to set aside face and the international respect that comes with it in order for it to secure for itself what it needs to survive.
China's tightening grip on the West Philippine Sea is a desperate hold on survival. It has never been about some historical footnote it suddenly remembered and only romanticized recently. It has never been about a sovereignty it never had. China is an old country and is as old as civilization itself. It has produced a number of great and learned men, none of whom ever made the strange claim its desperate contemporary leaders are making now.
Still another factor that has held China back is that it knows it is not the time to provoke a real shooting confrontation with the United States. China is aware that any mistake that can be made in the West Philippine Sea can only be its own. The US may be being provocative. But China knows only it can push things over the edge. And it is not prepared to do that. Yet. Make that one mistake, and China knows all it has been striving for to attain can be set back instead by decades.