Unions are labour cartels for the purpose of extracting above-market wages from the commons, a sort of mafia. They are incompatible with capitalism and libertarianism, especially with libertarianism.
What is rich people collaborating called? You might claim that it is incompatible with capitalism but it’s just a fact of it. It’s easier for a small number of resourceful people (and capital gives them resources) to collaborate than for many people with not much more resources than their house/mortgage. This is what Adam Smith told us anyway.
Simply assuming that "every employer does it because theoretically it is easier for then to do so" to help your argument is rather self-serving, considering that labour unions are explicitly legal and exist.
Cartels are not at odds with libertarianism. In fact, freedom of association is the fundamental underpinning of libertarianism. Unions are the libertarian solution to labour woes. Other groups normally favour regulation instead.
Libertarians don't have a theoretical problem with cartels because if a cartel tries to push for above-market prices, someone else will swoop in and start doing it for less, taking all the cartel customers with them.
By this logic, every corporation is a cartel to extract below-market wages from the commons. Both sides are bargaining collectively. And so you'd be saying both are incompatible with libertarianism.
There are strands of libertarian thought, I suppose, where government shouldn't be incorporating businesses at all. But it's still legit to say libertarianism is compatible with corporations and with labor unions.