Free "junky" content is popular simply because it's free and plentiful, and has a vastly, unquantifiably lower barrier to entry than paid content -- and not because it's dopamine-releasing junk. It's popular because it's accessible, requires little cognitive load to evaluate ahead of time, and its momentary consumption is noncommittal. If article mills stopped producing throwaway clickbait to fill an Outbrain footer, the corresponding "demand" would plummet.
But this content keeps being produced, because business models on the web suck, so every publisher feels they have to put some eggs into advertising. Because reader attention has an upper bound, one publisher's gain is quite often another's loss, and this directly translates into advertising revenue.
Given the low appetite for the masses to pay, Apple, Google, Facebook, Medium, and incumbent news publications have all tried to solve this problem in ways that further integrate advertising, analytics, and personalized suggestions, and these efforts continue to face challenges due to the misalignment of incentives.