Assuming the theory that tariffs pushes local manufacturing is true: We could produce our own stuff, but it takes time between the institution of the tariff and when a factory starts producing <item>, so there’s still going to be a lot of expensive stuff in the mean time.

More expensive steel (for example) means lots of things like cars get more expensive, which means fewer people can afford them…which means fewer cars to produce…which means less need for related industries (textiles, plastics, rubber, etc.), which (in general) means fewer employees needed to build those cars and supply those related industries…which means more people with less money…which means those people are going to buy fewer things, which means less money for all of the other businesses… and so on. It just spreads. Everything depends on everything else.

You can’t just build a factory and start producing say, steel, overnight. And what happens if the tariff is dropped right after you finish your factory? You’re going to get hosed, so it’s a huge risk - especially with someone as inconsistent as Trump.