Well, for one, that would require you to be able to have land to do so. I would be shocked if many of these people had an actual, freely available choice on the matter.
Assume you live in a "first-world" nation, how do you propose to become a subsistence farmer?
Buy cheap land out in the middle of nowhere and start farming enough to provide food for yourself and some excess to pay expenses. There is a reason people don't do that.