It's a result of how unions are structured in the US. (It's different in Europe from what I understand, and they're more popular there as a result.) In the US unions are structured as a monopoly on labor which then nominally has more negotiating power with the employer.

The problem is that monopolies are inherently an industrial scale fail factory and everything about them makes everything worse.

You then have effectively two possibilities.

The first is that the employer is competing with a hundred other local employers for labor. In this case there isn't really anything for the union to do, because the employer will already have to offer competitive working conditions or employees will go work for somebody else. And the employer in a competitive market won't have thick margins for the union to extract from, implying that the union's overhead will be larger than what they can extract from the employer.

The second is that the employer is, itself, a monopoly. In theory it now has thick margins that the union can extract concessions out of. But then you have two monopolies negotiating with each other. It's a battle of attrition. Can the employer afford to have their facility closed for longer than you can afford to be on strike? You know the answer to this is for large multinational corporations. The corporation will barely even notice the complete shuttering of that facility because they have a thousand more to pick up the slack.

A monopsony employer is still a big problem, of course, it's just not a problem that unions are a solution to. Try antitrust, or lower regulatory barriers to small businesses so that there are more competing local employers etc.