>As a result, the existing national unions have repeatedly demonstrated that they’re not interested in improving working conditions for their members.
It always comes down to this. And you can argue endlessly about the various failings of modern organized labor. But ultimately it's all we've got. The capitalists have demonstrated for centuries over and over again that left unchecked they will always take capitalism to a level of borderline subsistence for the working class to maximize profits. Your boss is not looking out for you; ever.
To have an organization with funding and influence that has at least professed to defend your rights gives a more realistic chance of having your voice heard, no matter how slight.