I think for most it's not a matter of "can't afford", but rather closer to "don't think it's a use of money that will benefit me over the alternative."
As a parallel: I can easily afford to buy a new car; I choose to buy used cars because it's a better use of money than the alternative.
I believe this discounts the very heavy anti-union propaganda that we've all been subject to since... forever.
I'm not even from the US (I'm from Brazil) but here, similarly, most have a very bad view of unions, fueled by employers that, of course, don't want to give workers any power.
It's always ironic when we see today's workers continuously losing rights that were achieved mostly through unions (with a fair share of blood) actively hate unions without ever truly looking into them.
Smart people always assume they are immune to propaganda, and objectively investigating where those feelings come from is not something most take the time to do. Confirmation bias plays a big role here.
Unions do have to overcome that history as well as the "free rider problem", but I don't think that's particularly unique to them versus any other "If you will give me money and power, I will make your life better" proposition.
We get advertised travel, lifestyle, nutrition, convenience/utility, and entertainment products all the time. Most of those things over-promise and turn out to underperform their promises (yet not their price tags) and so people who are generally happy with the status quo need some activation energy and convincing to give up money and power now in hopes that this new thing being advertised/promised to them will deliver on its promises. That seems entirely rational to me.
As a parallel: I can easily afford to buy a new car; I choose to buy used cars because it's a better use of money than the alternative.
Many engineers are rational optimizers by nature.