Specifically, during the Reagan Administration, the official policy on how to interpret antitrust law shifted from one major economic school to another—I'm not fully up on the details, but my understanding is that the latter is the Chicago School, and that a big part of the shift is to focus not on how much of the market the company dominates, or whether a merger will be bad for competition or employees, but rather to look solely at consumer prices.

This is a terrible metric to use as a single guide, especially when it is also (in the case of mergers & acquisitions) focused solely on the immediate aftermath of the merger.

Lina Khan was starting to push a shift back away from this deliberate giveaway to corporate interests, but then Trump was elected again, and any hope of that went out the window.