America has done nothing of the kind....Something which America has promised Taiwan, in the event they declare independence from China, officially!
In fact, America has been deliberately vague in defining what it's military commitments are to Taiwan as it fears that clearly defining that relationship would lead to war.
If it said it will guarantee Taiwan's safety no matter what, the next pro-independence candidate to get into office will promptly declare independence on the back of that guarantee, and China would invade.
If it said it will not come to Taiwan's aid, then China will attack.
Or so the thinking in Washington goes at least.
In reality, what America's military obligations are to Taiwan is to be defined as best serves America's interest. As things stand, America's best interests are served by remaining vague. But if that status quo is broken, either by a declaration of independence from Taiwan, or a PLA attack, then the American president at the time will decide what America is obliged to do based on the projected outcome and cost of getting directly involved in a shooting war with China.
If it looks likely that America will loose, or only win at unacceptably high cost, then the American president will in all likelihood declare that America never had any obligation to come to Taiwan's aid if attacked, issue all the usual condemnations and urge an end of hostilities and secretly hope the war ends quickly so they present China with 'reset' button (hopefully with the correct translation this time) and get back to business as usual before too long.