American unions are kneecapped by the government. The 1947 Taft-Hartley Act made solidarity strikes (and several other forms of labor protest) illegal. It also opened the door for states to enact “right-to-work” laws.
This law is still standing in part because US courts have been anti-labor for their entire existence, aside from a brief period during FDR’s administration.
Important to note that this is a workaround. Solidarity strikes (which normally include general strikes) are illegal, but there’s no law that prevents every union from happening to strike on their own behalf at the same time.