With shitty climates. The surface area of optimal climate in the US is likely larger than that of all the countries you've named combined. What's optimal? For example in Canada most farmed areas yield only one harvest per year. And most of the land is barren wasteland. You have some of it as well, but it's not the majority of the territory.
Geographic determinism is one of those theories that explains everything after the fact and predicts almost nothing
Argentina seems like a counterexample here: the Pampas are one of the world's major temperate agricultural regions.
Brazil is pretty damn fertile, and nearly as large as the US though... If we're just talking about farmable land area, I'd be surprised if the US is larger than Brazil. Farmed land area is a different thing though. Not sure how what compares.