A company with 100 employees losing an employee costs them ~1% of their revenue until a replacement can be found.

An employee losing their employer loses ~100% of their revenue until a replacement is found.

The two are nowhere near equivalent.

On the other hand, when employees unionize, a company with 100 employees stands to lose ~100% of their revenue if employees choose to strike. That puts the two sides on an even footing.

And when companies coordinate to prevent that? It doesn't even have to be explicit behind the scenes coordination. Companies that base your salary offer off of your previously salary are still coordinating even though they don't plan it