I'm trying to parse the idea of "a collective agreement" but can't fully wrap my head around how that would work.

It seems to me more like the lack of a "Walmart Law" is a result of e.g. lack of economies of scale and other economic structure, rather than some collective agreement. (If it was profitable to break out of that agreement and start a "Walmart Law", it seems we'd see that happen pretty quickly?)

But if you know more about this and I'm off the mark I'd love to learn