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.
You think you're thinking about reality.
All you're doing is imagining reality. You're living in a fantasy.
It’s news to me that what Adam Smith wrote about in that book was theoretical. Oh well.
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.
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.
This statement makes only sense if you don't know what markets are, how they work and what their purpose is.