I get the impression you're over-simplifying this and grasping for whatever assumption you need just to try to make your point.
While some of us have run with my oversimplification, there are plenty of comments in this sub-thread about down-selecting. Your answer to that is a giant assumption on what the article (which again is rigorously sourced) 'implies'.
> The article also implies other contractors are also listed, although they don't get similar preferential price treatment.
In fact, the IG report[0] that is cited in the selection of the article I replied with has nothing to do with McKinsey, but a different contractor altogether. Reading comprehension is your friend here: "In 2013, the GSA Inspector General traced a similar situation with different contractors."
Agencies aren't necessarily selecting the more expensive item. As you point out, in most cases they have a mandate not to. They're awarding the bid to an already-short list designed to generate the highest IFF possible for the GSA.
Which is what the article's about.
[0]https://www.gsaig.gov/sites/default/files/audit-reports/A120...