Because it's legal and their status is protected. In a free market there'd be a market for unions too.

Effectively the fees are an insurance payment.

Many big unions have strike pay, in order to give more tooth to the strike threat, otherwise the company could just wait two weeks for the people to go broke. You could argue that people could just save money, but the same can be said of health insurance, yet that too is a business model.

You pay money in, and if you ever actually strike, you take money out from the pool made by you and a thousand other businesses. It also pays lawyers to make sure that strike operation is perfectly on the level and legal, and has no one arrested.

As for the salaried bosses... Someone is a representative of the union. If an employee gets a complaint from management, some union person needs to assess if the union of employees will protect them or not. That is an additional responsibility, and additional responsibilities should be paid for. Same with wage/benefit negotiations.

That does not mean unions are a good or a bad thing, just that their business model has been tried and tested, and evolved from older models into the one we see today because a union, like anything in this world, is a product to be sold, and they do cater to their consumers, but do need to be paid for their product.

Good and bad union environments exist. They're a lot like governments: Good and bad exist, and the best ones are the ones whose touch is felt the least by all the good members of its society.

They always have been useless. The first ever act of a union I'm North America was a lynching, the second was a child kidnapping. They're nothing more than a mafia.