I believe that $77B as a fairly good cause for investors/other companies to set up shop and make investment into the US semiconductors market. According to random market statistic site, it is also currently growing by 10% each year.

Is that market so poor that subsidies really is needed on the basis of profitability?

A $77B market that's growing by 10% each year doesn't mean much when the government turns against you demands you do a bunch of expensive investments for nationalist reasons. See also: all the investors fleeing china because they were spooked by the tech crackdown/covid lockdowns. "Punishing a company because it's selling out Americans in favor of cheap labor" is basically the same thing but with a different coat of paint.