One of the key bits of fermentation is that the conditions need to be pretty close to perfect for yeast to be a dominant Factor. You need to have a relatively sterile environment, you definitely need to make sure that there's nothing else competing, and you need to make sure that you're within certain factors of things like acidity. Stomach acid has a pH of 1-2, most brewable juices are closer to 3-4.

Some microbes do end up surviving the journey through your stomach, and they become the microbiome inside of you. I think that brewer's yeast would find that environment extremely challenging more than the stomach even because that microbiome is like skid row, filled with the dregs of the food you eat. The spoiled rich kid brewers yeast isn't going to thrive in that environment.

Consider a piece of advice from cider brewing: you are sometimes told if you don't have yeast to just juice the fruit without washing it and natural yeast on the fruit will grow in your cider and if you're successful it'll taste better than beer brewer's yeast. This shows that yeast is everywhere and it's on many things we might routinely eat and people aren't going around naturally drunk after eating an apple off a tree or some berries off a bush.