Degrowth

Right but for that you need to grow in different places. The prairies where all the grains are grown are not suitable for much else, besides maybe soybeans. We already grow way more soybeans than humans need and use them as livestock feed.

Feeding everyone just soy and rice is not exactly great. The highly varied diet that vegans eat tends to include a lot of fancy nuts like almonds and cashews, avocados, tropical fruit, wide varieties of greens, olive oils, organic everything.

A lot of that stuff is already quite expensive. There’s a ton of incentive for farmers to grow it because it would be way more profitable for them than corn. They don’t. Why? Because they don’t have the soil quality, access to water, and warm dry climate you need to grow the best stuff.

Those last two are the key, and why California is such a powerhouse. You need the dry climate because a humid climate has too much rain, causing all kinds of diseases (bacteria and fungus) that destroy your expensive crops. But you also need a lot of water for the plants to grow. This means you’re using a lot of irrigation in a place with not much rainfall, so you have to bring in the water from somewhere else (by river or pipeline).

Pretty much all of the North American Midwest and prairies are classified as humid continental climates. This makes these areas (where corn is grown) quite poor for growing a lot of the expensive crops grown in California.