You can’t just say “hurt durr Americans are just irresponsible;”

Actually, I can. That's why americans are more obese and have more credit card debt - it all boils down to irresponsibility.

I’m trying to have a conversation about what it would take to actually solving the problem here

Solving the problem would be rather simple, subsidize healthy food so a cucumber doesn't cost 6$. Make mandatory cooking classes in school so kids know how to cook, at least to some extent. The magical word is "education".

even if the problem really were that Americans were more irresponsible on average compared to people from other countries

There is no "if". They are.

there would have to be a systemic reason why

No. It boils down to people eating shitty food and not knowing/having no interest in knowing how to cook.

Like, we can argue all day about american processed food being full of additives and sugar and high fructose corn sirup etc, which is certainly the case, but at the end of the day, the people who eat it know that this is the case, so they are irresponsible, and if they don't, it once again boils down to education.