But growing soybeans year after year after year doesn't work very well, ecologically speaking[1]. Crop rotation is essential for soil and crop health. For a whole lot of practical reasons, animal feed is more efficient to add to the rotation than, say, a fruit crop. Good or bad, agriculture does not operate in one dimension.

[1] Source: I grow soybeans for the human market. To which I'll add that human grade soybeans themselves are less efficient to grow than animal grade soybeans. They command a pretty sizeable premium to reflect that added cost of production. It's not exactly as simple as shifting your diet to eating animal feed.