Sarah ~ You have some valid points in your post. I have read many other posts that have the same theme ie check out the whole foods section at this site. We have members here that talk about 'eating clean' quite often.
Just last night, a group were talking about avoiding the 7 C's: I think they were Colas, Candy, Cakes, Cookies, Crackers, Cereals, and Ice Cream; and only eat things grown or shot (animals). But there were lots of exceptions, like dairy products. Some people will like the idea; some won't!
I eat 'large flake' oatmeal or other whole grain cereal, whole grain breads, a few dairy products like cheese, yogurt, and skim milk, olive oil, home grown veggies, fresh and frozen fruits, lean meats and fish, etc. Occasionally, I may have a few whole grain crackers, and a very small amount of ice cream as a treat.
North Americans do eat a lot of greasy fast foods, but many people here are trying to quit that. They say the French and Asians are slimmer than us too; but I live in Canada, and just as many French people here have weight problems as other groups. While I do think our eating habits really need an overhaul, I also think it is our lifestyle choices as well. We have long, cold winters here; and many people hate to even go outside becuz of that.
You have an advantage to have decent weather year round and access to fresh veggies, fruits, and fish year round as well. I just think there are many more factors at play here; not just packaged cereals. For me, how much I ate of the not-so-bad foods, was a bigger factor.
I understand your concern though ~ from your vantage point, it may seem that all we eat is packaged and artificial stuff. I don't eat a lot of packaged foods myself; and we by-pass those cookie and chip aisles in the store completely, sticking to the outside sections mostly.
For some, trying to change life-long habits will take time; but thanks for sharing your thoughts ...
ROSEBUD