Recommendation units almost always appear after the article body. The position looks like an afterthought and is a deliberate trade between two things a publisher cannot have at once.
The page has two audiences with different values
A reader partway through an article is engaged, and interrupting them risks the visit. A reader who has finished has already delivered the value of the page.
Publishers therefore protect the middle of the article and monetise the end, where the cost of a poor experience is lowest.
The same logic explains why the units are dense at the foot of the page. Nothing is being interrupted, so quantity is limited only by appearance.
The exit point is the only moment of surplus attention
A reader finishing an article has to decide what to do next, and most would otherwise leave the site entirely.
A recommendation unit intercepts that decision, which is why it works at all despite sitting where fewest people look.
It competes against nothing except leaving, which is a much weaker competitor than the article the reader came for.
Why the inventory is cheap
Only a portion of readers scroll far enough to see the unit, so the effective audience is a fraction of the page's traffic.
Advertisers know this and bid accordingly, which caps what the placement can earn per visitor.
Cheap inventory attracts advertisers with low margins per conversion and high volume requirements, which shapes the mix of what appears there.
That mix then reinforces the position, because a unit filled with low-consideration offers is one no publisher wants placed higher up the page.
Mixing paid and editorial recommendations
Most widgets contain both the publisher's own articles and paid placements, usually in the same visual treatment.
The editorial links raise the overall click rate of the unit, which lifts the value of the paid slots beside them.
They also keep readers on the site, so the publisher gains twice from a unit that appears to be purely an advertising product.
What changes the placement decision
Publishers with subscription revenue place these units less aggressively or remove them, because the reader experience carries a direct financial value.
Sites monetised entirely by advertising have the opposite incentive and push units higher up the page, sometimes into the article body.
The position of the widget is therefore a reliable indication of the publisher's business model, readable before any commercial conversation takes place.