I have no clue if the assertion is correct.
Edit: FWIW, it appears that US soy is mostly used for livestock:
> Just over 70 percent of the soybeans grown in the United States are used for animal feed
https://www.usda.gov/sites/default/files/documents/coexisten...
And globally, even a higher percentage:
> About 85 percent of the world’s soybean crop is processed into meal and vegetable oil, and virtually all of that meal is used in animal feed.
https://web.archive.org/web/20170112075924/http://www.soyate...
There’s an immense amount of green things growing we can’t digest that cows, sheep, goats, and chickens will happily chomp on or peck at. The idea is that we then eat the cows, etc. It’s simple really.
As an aside, I’ve been on a fat-loss diet all lockdown long, and if it wasn’t for animal products, it simply wouldn’t have worked as well as it did. I need high protein within a limited calorie budget. Hello, chicken breasts and whey powder.
Anyway, if cows are not a good enough system to feed 7 billion people, we should grow fake cow parts in vats, also neatly eliminating the need to kill the animals. I’d be the first to sign up for a fake steak, if it was chemically identical to a grass-fed one (the taste of which is fantastic, absolutely beyond description)
Seriously though the reason it is fed to animals is likely because of cost. Generally animal feed is chosen based on cost and effectiveness.
Really though, humans can't eat grass, beet pulp shreds, beet molasses, and many other things we feed animals.