I've seen this point before but is there much reason to believe this is the case? Unhealthy food is often priced the same or more expensive than other, healthier food, especially when buying large amounts. Where I live, it is much cheaper to buy fresh produce (fruits and vegetables) and raw meat than it is to buy frozen pizzas, chips, take-out food on a regular basis, chocolate, breaded fish sticks, microwave meals etc.

I think another reason could be that people become conditioned to like things that make them feel good and happy, and something that tastes "nice" (i.e the engineered foods made to tap into our love for salt, fat and sugar) is a good relief from the stresses that come from being poor. But I figure this is an unpopular hypothesis because of the subjectivist-individualist ideology in which only the very last step in making a choice is relevant, rather than the reasons leading up to the choice. So we throw subjective preferences out and attempt to decide if buying such food is "rational" or "irrational" at the point of purchase. This economism unfortunately shuts out important details in preference formation.

What about the role of education, advertising, the pressures faced by poor parents in their actions towards their children etc.?

Agreed.

Fresher foods also often have a shorter shelf life and are harder to store, which is a particular issue if getting to the store often is a struggle (e.g. public buses).

You then combine that with lack of cooking instruction in many low income public schools, and you have a recipe for ready meals and similar convenience foods.

Or the opposite.

You eat bad food, it decreases your intelligence or at least your ability to function effectively (bad digestion, blood sugar spikes, general longer term health issues).

Then you spiral down.

Fresh food isn't expensive. On the contrary; it's extremely cheap compared to the benefits it confers.

It's far better value for money than, say, another 5 sqm on your living quarters.