That's assuming the quality and quantity of missiles are the same.
China will hit the important cities of whoever aids US aggression. Thus massively damaging US' economy, especially since like 95% of semiconductors are imported from Asia.
But the equipment level isn't the same. One side can make way way more missiles/seaborne drones.
If you're US planning an attack on Taiwan, you're faced with two different options that are each very disadvantageous in their own way:
1. Somehow involve tons of Asian countries in the attack. In response, China will massively bomb those areas and then counterinvade. To have a hope of preventing that, US must institute a general draft and then land in Asia with huge amounts of war material (how will it be produced?), using its own forces (and most vitally, air defenses) to protect Asian cities.
2. Go at it alone. You however only have up to a dozen bases at most. All your war materiel needs to be routed through these limited logistics chokepoints.
The reason US has not attacked yet is because these are pretty severe problems that their strategists can't solve. Mind you these were issues that US faced even back in the third Taiwan strait crisis.
