There was a cherry tree in the backyard of a house I used to live in and every year the fruit would get funky and ferment. Birds and squirrels would make a habit of eating it and the backyard would be filled birds all flopping around and squirrels trying but mostly failing to find their way up trees. They knew what they were getting into, and they indulged. For about one week a year it was like a frat house out there.

I can imagine people discovered the principle there, but brewing on a more industrial scale is necessary to provide a steady supply. Just about anything can be fermented, but that proecess takes time to do properly. It's a form of farming.

So yeah, you can forage for it, but you'll get so little it's barely worth the trouble.