Because manufacturing jobs are never coming back, short of WWIII starting. Automation has seen to that - we won't waste human beings on something as dumb as pulling levers 1000x a day.
But if you mean "value created" then manufacturing never left [1]. The US simply moved up the value chain, producing more complicated and useful technology with fewer workers and more automation.
Both China and the US understand that workers' days are numbered. The future belongs to robots and the folks who know how to program them, not workers who directly operate machines. By 2050, barring any really stupid changes in immigration policy or women's rights, both nations will be about 500M people - but the US will be more productive per citizen.
1: https://www.macrotrends.net/global-metrics/countries/USA/uni...
Macrotrends is a spam site that is very well-indexed. I have seen it post blatantly, unbelievably false stats all the time that i have developed a blindness for it in my google feel, i automatically scroll past it. Yes most of the time numbers there are correct, but it can't at all be trusted.
And well, there is no way that either China will shrink to 500 M by 2050 barring WWIII, or U.S. grow to 500M unless there is universal, free-for-all, open border policy in place.