Do you not purchase insurance? Same principle. Collective bargaining and buying power unlocks things even you, a wealthy above average human, might wish to have affordable.
> Why should I as a non-unionized worker today join one?
So they continue to exist and rules don't slide back further.
I mean if you are old and rich enough enough not to care about the next decade of working life then good for you, I guess. But society could very well be ripped apart by the very things that the tech industry is rolling out, and unions are one of the few things that stand in the way of absolutely massive regressions in the way humans who have the temerity to want to eat will have to tolerate being treated.
Personally I want to see them survive, and non-union workers joining them when they can is one of those ways that can happen.
(As an IT freelancer I cannot, meaningfully; I assure you that when my work pattern changes I will)