That's not strictly true. In the US there is in most states NO requirements for paid leave outside of a few protected types. But not vacation or sick. And we also are largely not unionized. We still have time off albeit MUCH less than Europeans and such.
Though I'd argue the norm of having any paid time off is a byproduct of labor/union battles in the past.
I'd say you earned them as part of the agreed compensation in exchange for your work. That way you also cover circumstances where there's no union, and no government requirement, but the employer still offers PTO to be competitive with other employers. Which theoretically could also be driven by other people's unions or governments, but then that would be indirect.
Besides which, regardless of how the arrangement came to be, you earned your PTO.