- In the US we have vast surpluses of corn, and it's heavily subsidized by the government.

- Feed lots can fit a whole lot more cows per acre.

- Corn tends to fatten up cows more (and more quickly) than grazing.

- Increased fat content = meat that grades higher and tends to sell for more meaning you also earn more $/lb on top of the other reasons.

There was a policy decision made in the Nixon era.

Basically, it was determined that corn was the cheapest way to generate certain caloric targets. This achieved that objective and also secondarily helped with certain political objectives. So there's a whole subsidy and trade system around corn growth that basically stifles everything else.

The outcome for agriculture has overall been devastation for all but the largest operators. The "scale up" makes it impossible for all but the largest farms to compete. When my parents were kids in the 60s, a diversified, smallish farm was a sustainable thing. When I was a kid in the 80s/90s, they concentrated on bigger dairy or corn operations. Today, they're going out of business or make money selling doughnuts.

US subsidy policy encourages huge over-production of grain, so ways have to be found to get rid of it such as feeding it to animals or making ethanol out of it, or making HFCS and putting it in drinks.

It's the farmers trying to raise too many cattle on too little land.

Also feedlots.

I had to google feedlots. They seem a tad unsustainable.