In a world of atomized workers, even the ones who can run fast on the treadmill are only good as long as they can run fast. If the treadmill changes or they get tired they're out.
Unions help because they have political power. They can make it so that societies are also about people, not just about efficient allocation of capital.
So you're either disenfranchised or you're a billionaire or you're part of a group like a union that can raise cash and use it for things you care about. That's a good argument to be in a union.
Or it would be, except (apart from a few specialist groups like police unions) unions seem to be amazingly bad at actually doing this. We haven't had pro-worker legislation in decades as far as I can tell. I don't know why unions are so bad at this as (at least IMHO) it seems like a core responsibility AND a major selling point. But they are. We've seen fewer and fewer rights for workers, higher taxes for working people, the removal of benefits for working people, higher student loans, sky high house prices, you name it.
Unions were never as powerful as the capitalist elites. All worker rights were hard-won (often with the price of life) and there has always been a lot of counter-pressure from capitalists. For a while, following the New Deal (in the US) and WW2 globally, workers had a social context that favored them a little.
Starting with the Reagan-Thatcher era though, political power has shifted rapidly in favor of capital. Neo-liberal ideology has become the dominant intellectual lens through which ("well educated") people evaluate the world. Policy makers have likewise shifted greatly towards the right-wing. New Labour in the UK [1] and the New Democrats [2] in the US are essentially center-right groups. Actual leftist positions are considered fringe in the Western World nowadays. This has in turn greatly affected the political power of unions, which are historically left-wing movements.
Leftist movements can gain ground as long as the people with actual power are somewhat amenable to their goals. This hasn't been the case in about 4 decades.
[1] https://en.wikipedia.org/wiki/New_Labour [2] https://en.wikipedia.org/wiki/New_Democrats
But here we are. Joining a union won't help me in the local or immediate sense and it won't change politics. I don't like the economic or political system. But with zero options to change it, I have to pursue whatever will get me the best return for my efforts. That isn't unionisation sadly. Its a 19th century solution that doesn't seem to fit 21st century problems. :(
Also, if the whole industry is being offshored or replaced with AI, the union won't be any use as they'll have no leverage either...