The basic reason this happens is that people's wages are only worth what they can buy, and in order for them to be able to buy those things they need to be produced and shipped. Sure, you could argue that this doesn't actually matter in practice because the corporations will always have enough excess profit to just pay people higher wages and employ more people to do less when the unions demand it, and that they'll have to do so rather than increasing prices, and that these cuts in profits will somehow always come out of the pockets of unsympathetic billionaires rather than ordinary workers' pension funds. That requires a lot more about the world to work out just right compared to the economics 101 version though.