the first one was in 1982 or something, so you have nearly 3 whole generations who were exposed to it (X, Millennial, and Z). I really can't tell if you're actually incredulous; because all the nutrition stuff is told to schoolchildren. Adults don't use a chart, they use self-help books.
The first one came out in 1992, and was active until MyPyramid came out in 2005. Which was then active until MyPlate came out in 2011.
My main point was (i think!) that really the only people seeing these posters on a regular basis are schoolchildren. I think i've seen the pyramid a dozen times in the last 20 years, on cereal boxes or websites or whatever, but if you don't recognize it, it's easily written off. Maslow also had a pyramid, etc.
In the late 90s I was in high school in a town with less that 80k people in the middle of the congenital USA and the pyramid with bread, cereal, rice and pasta at the base was not seriously pushed, or taken seriously, at school or when it came up outside of school.