You can only eat fresh corn when it's in season, and only within a week or so after it's been picked. To be a suitable year-round survival food, corn has to be processed into corn meal. This usually entails treating it with lye, drying it, and grinding it. This gets you corn meal, grits, polenta. Or hominy if you skip the grinding step.

We don't have the equipment to do this at the necessary scale. Corn just isn't used as a survival crop this way, so there's never been an economic need to invest in the machinery to do it.

We probably do have the infrastructure to keep producing wheat bread in mass quantities. Do we have the peanut production and processing infrastructure to handle peanut butter becoming our primary protein source? I doubt it.

And of course with soybeans... they would need to be dried or canned, packaged, stored, and shipped. Doable but different from what we are currently set up to do.

And of course there's the psychological impacts of all of that, which again are probably oreferable to starving but wouldn't be good. People fight wars (or gang wars)over this kind of thing.