We bought it in the same stores where we bought real food back then. We buy food in the same stores that have breakfast cereal now.
I haven't watched TV or movies for like the last 10 years, and I've blocked ads on my computers for ~20, so I've at least minimized the most blatant exposure, but I don't think myself immune. That's why I've done what I can to remove them from my life. But I'm naive too; like I didn't realize until recently that radio "callers" are just iheartmedia employees, or that you can just buy an "interest" piece on the news or Ellen or an "opinion" or "lifestyle" piece in the newspaper or whatever. It makes sense in retrospect, but the extent to which literally all media around us are just ads is hard to wrap one's head around, and a little unexpected IMO. I don't think it's intuitive or that you have to be dumb to be tricked. You just have to be honest enough that it wouldn't occur to you that everything around you is lying and that these people will relentlessly work to construct some Hell version of Plato's cave in order to sell you things and that it's basically legal to do so.
Maybe I'm just one of the dumb ones, but IMO ads like this[0] masquerading as national news should maybe require extremely clear labeling and disclaimers, or just be illegal. Maybe when shills on youtube say "this is sponsored, but this is my real opinion", the second half of that sentence should be illegal. Maybe they should have to say "this video is an advertisement for X, and I am not presenting my opinions on it".
The food desert idea is plausible, but the literal definition is useless (poor people can't walk 0.5 miles?), so I'm forced to be skeptical of any claims around it.
To me the plausible explanation for breakfast cereals is that people underestimate how evil these companies can be, and probably figure it must be illegal to sell candy advertised as food or something, so it can't be that bad if it's so common and if it's allowed to be advertised on TV. Surely they couldn't or wouldn't say it's "part of a complete breakfast" if it weren't at least mostly true. Surely if it's on the news, the reporter would mention if it's actually extremely horrible for you and surely the "report" isn't literally written by the advertiser.
[0] https://www.foxnews.com/food-drink/oreo-os-cereal-returning-...