I think you're severely underestimating the lack of resources/materials that comes with literally everything being torched by nuclear fire, and then deteriorating over 200 years. You're also probably overestimating how smart the average wastelander would be considering very few of them would have access to even the simplest form of education. I think it is safe to say the games have a lot of guns only because it'd be boring otherwise. Realistically, there wouldn't be much to go around.