Unions in the United States are entrenched with a particular government-backed structure that puts the high-ups far from the working guys... they're not all necessarily leftist or willing to accept models other than management-and-bargaining.
A lot of people believe the sanctioned union structure was a compromise with employers who did not want fully functional collective bargaining counterparties but who knew they couldn't eliminate unions entirely, and they could be right as far as I know.
An argument in favor of this viewpoint is the treatment of sectoral/sympathy strikes (and other important solidarity tactics like sectoral boycotts) under the Taft-Hartley act: https://en.m.wikipedia.org/wiki/Taft%E2%80%93Hartley_Act
People think of the NLRA as a legacy of the New Deal and therefore inherently pro-labor, but conveniently forget that the deal was changed by a bipartisan alliance of corporatists over Truman's veto.
Sectoral/sympathy strikes were historically the backbone of union power in the US and remain so today in jurisdictions with strong unions. Legally protecting ineffectual strikes but not effective strikes traps labor in a local maximum, where there's just enough to lose to disincentivize going for more substantial gains.
