There's an equal and opposite level of messaging to keep eating, which is less conscious and runs under the radar.
Snacks are literally designed to be addictive. TV ads start from birth. Most restaurants have huge portion sizes. "Family" and "Festival" events assume overeating is expected. Junk food is cheap and quality food is expensive. Overeating is framed as being "naughty" but also indulgent and nurturing.
All of this is a huge social problem that's not acknowledged at all.
It's very, very hard to Just Eat Less when there's a constant barrage of messaging encouraging you to do the opposite, and you're not even aware of it.
The contradictory messaging is actually a classic crazy-making psychological double bind. So of course it's very difficult to make a dent in this, and even harder to permanently change habits so all the contradictions no longer influence you.
ppl keep blaming this, but this is contradicted by shrinkflation, yet people still are getting fatter than ever. There is nothing to stop someone from buying more food to offset smaller portions.