Because the country has lower standards of living and a government that busts independent unions while deliberately manipulating their currency to make sure workers do not end up with too much buying power.

Cost of living is somewhere between 40% to 70% less in China.

That is a bit circular. The economy isn't like weather where some places just have less rain. China's cost of living is 40-70% lower because the people doing the work are willing to charge less.

In some cases, because we make it illegal for Americans to act on it even they were willing/interested. That’s not the majority of the story, but it’s part of it.

Why are American workers not willing to be packed into a 8 people dormitory and works 12 hours a day for $1000 a month?

Probably because the American couldn’t even pay for housing on wage the Chinese worker can.

Because Americans have more options and opportunities. The job market is better, simple as that. This is a big reason why people emigrate.

Different places have different costs of living.

Unfortunately, higher cost of living doesn’t automatically raise wages though.

While true in the abstract, that's not germane here.