Good point, Roy.
Under common law principles, contracts were only valid if both parties agreed to exactly the same terms, without any fraud or coercion. That meant that employees could SEEK to bargain collectively, but equally that employers could decline that offer. The National Labor Relations Act, one of the most egregiously authoritarian pieces of special interest legislation ever enacted in the US, changed that. It destroyed the neutrality of the common law in this regard by mandating that employers must bargain “in good faith” with union representatives. (It also made it illegal for workers who don’t want union representation to negotiate their own deals.)
Unions are thus backed up by the threat of government coercion and have exploited that unique privilege to the maximum.
