Collusion of this type has a very minor impact on market rates. The example you cite is tech companies coming to an agreement to not cold call each other's employees to head hunt them. And wages in Silicon Valley have exploded over the last couple of decades so obviously there are much larger forces at play in setting wages than these collusions.

This sort of collusion can also be addressed in a much more targeted way than creating laws that give workers who unionize control over major hiring decisions of their employer.