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.

Unions were very weak in the late 19th century, because the only tools at their disposal were extra-judicial violence, which was met with state retaliation. During that period, when contracting rights were at their strongest, workers saw wages grow at the fastest rate in US history.

Unions in the private sector became much weaker over the last 40 years, but that was not due to the labor laws that empower unions being eased. It was due to the type of industries that unions are prone to form in (e.g. labor-intensive, high volume manufacturing) contracting in the US.

The laws that empower unions make the US inhospitable to key industries.

What's going to happen to the US electric car manufacturing industry if Tesla's workers unionize?

What's going to happen to Amazon's business units if they see massive unionization?

The consequences for a US-based operation when their work force unionizes is a major disincentive for investing in production in the US, and a major impediment to existing US-centred companies from expanding.