Capitalism can generate more wealth and faster advances in living standards than any other economic system yet discovered. But it doesn't share that wealth and those advances naturally. Bargaining power is a crucial mediator in capitalism, and workers often end up with too little of it — and if that imbalance persists, then the entire system is imperiled, as capitalist systems can't survive without the support of the working class. Unions are one way to correct that imbalance. They've often saved capitalism from itself, and done so over the objections of the capitalists.
That's partly why you'll find unions pretty much everywhere you'll find capitalism. But right now, in the United States, unions are badly weakened:
