Because capitalists love to shift the blame.
Unions gave bargaining power to the workers. This drove up the wages of the workers a bit. But even without that, over time the wages of the workers increased relative to that of overseas. At some point they would have gotten to the tipping point where non-domestic production + transport was cheaper than having it made in detroit. See graph related.
This cycle continues everywhere. Locally and internationally. Once a place gets a lot of manufacturing, a lot of real money goes there, or a lot of currency needs to be bought to pay for that production. This increases the real total value of all the money in this area. These products also need to be sold elsewhere, not just in detroit. So the wages in detroit, and the united states as a whole, steadily increased until they real cost of production domestically was higher than overseas and transportation. At that point all the companies leave and build new factories overseas. Then the now abandoned areas slowly drain of money due to either spending money within the country so theres less, or importing and devaluating their currency. The economy continues to decline, making imports more expensive and money more scarce. Unions have to make consessions because they need the factory for the workers, they cant get it all in capitalism. So unions decline in power, their members stop paying, factories continue to leave.
And all of this despite all the factories the workers build, the real productive ability there was. It isnt that people got lazy, or didnt want to work, or were gibsmedats. All of this could come back if the workers had the power to build it again, but they don't. The capitalist have the power, and they dont care if they have to move, the love it. They can go wherever it is cheapest, and threaten the hard working people even more by threatening to move overseas. They can squeeze the workers for all they have got, they can even get the local governments to pay for the factories or give tax breaks, which happens all the time, such as here .
This all happens because of production for profit. The people need cars, they want to have cars. They can make cars in detroit, they can farm food in Michigan, they can sow clothes, chop logs, make pencils and paper, build houses and fix roads. But they can't do it because the workers need to eat, and to eat they need money, and to have money they need to work for somebody, and if they work for somebody they cannot fix the roads, they cannot build houses, they cannot sow clothes, they cannot mine rocks for construction, or mine coal for electricity.
They got us by the balls and they fucking love it.