Unsure what the solution is but I suspect some folk are time poor, and feel buying from a "junk food" provider allows them to cope.
And again some will just not be confident in being able to provide or even envisage what would be a healthy meal so can not face trying. Do they do home economics courses in schools these days?
I suspect 'carrots' rather than 'sticks' (no pun intended) help those who'd, deep down, like to be helped. And others, well what they find helps them cope with life ought not attract punishment from those with the power to make folks lives miserable.