The idea that it 'levels the playing field' is an opinion. I reject that opinion out of hand. If a company's workforce unionizes, a company cannot do much but pay the wages they demand. That is not a fair playing field. That is a cartel. Cartels are bad.

>After all the boss already has the power of a large organization

Poor representation of the facts. A boss has the authority to buy labour as he sees fit, and to offer a sum for it that he thinks he can afford. He does not have the power to force an employee to be hired at a price he wants.

> If a company's workforce unionizes, a company cannot do much but pay the wages they demand.

If a company’s workforce unionizes, the single employer and single collective bargaining negotiator are on much more equal footing than if the workforce does not unionized.

The shareholders whose interest the firm represents are already a cartel through the formation of the firm.

The concept of a cartel continues to elude you. Shareholders do not band together to increase the price of a product. They pool resources to produce a product.

> a company cannot do much but pay the wages they demand. That is not a fair playing field. That is a cartel. Cartels are bad.

This would be true if there were no negotiations (there are.)

Or if workers were all paid the same (they aren't).

Workers have a cartel on wage just as much as a company has a cartel on jobs. Companies use their leverage (aka a cartel) on workers - so to combat it they organize.