You've set up a strawman. There is no requirement that for a union to behave the way you suggest anywhere that I'm aware of. A union will generally be expected to provide certain protections, but there's a vast gap from having nobody to help you to reasonable representation is provided to even your positions vs. your employer to making the worst employees near impossible to fire.
E.g. the UK has plenty of strong unions, yet firing UK employees if you have good reason is shockingly easy for the most part seen with Scandinavian eyes. Some unions here are harder to deal with than others, sure, but it's also clearly possible to have unions that provide effective protection without making it impossible to get rid of people who do a bad job.
Part of the problem is that "doing a bad job" is sometimes used to hide cases of getting rid of people management doesn't like. It takes very few abuses of trust like that before you have a hardline union on the other side of the table that will not give a shit what arguments you have because they don't know whether or not the employer is telling the truth.
Once a relationship like that has soured, fixing it will be incredibly hard.
Remember that lives literally used to get lost over workers rights in the US (and still are many places in the world) - many unions today have a long institutional memory of people who died trying to gain concessions for their members.
E.g. AFL, now a part of the AFL-CIO was the instigator behind May Day as the international day for workers demonstrations, in part to continue the demonstrations for the 8 hour working day, in part in tribute to those who lost their lives in the Chicago Haymarket Massacre.
US unions in particular used to be on the forefront of this, and bore some of the worst brunts of violence, and many of the older US unions early became very much hardline for good reason.
Compare to unions many places in Europe where electoral success for left wing parties meant many of their demands where implement by law, allowing many unions to take a much softer approach and taking a lot of contentious issues off the table for union<->employer interactions because the hard stuff was dealt with by government.