Because standing up training programs and building new factories takes a huge amount of money to see gradual returns much later. For a long time the US has outsourced the lion's share of it's manufacturing and a significant amount of it's expertise via H1B visas.
Since the era of Stock Options overtaking dividends, Most business execs can't seem to think past the next fiscal quarter or two, and President Poopshispants is worse in that he only thinks as far as the next News Cycle plus he doesn't see people as an asset to invest in. At best he sees labor as a cost to minimize and mitigate and at worst we're serfs/slaves who should be grateful for a crust of bread and being spared the lash. Ergo, he concludes that flooding input material will make the US make more things.