Under current DoJ antitrust guidelines, there's nothing to stop a future administration from reviewing any anti-competitive actions ignored by the current one as part of an anti-competitive series of actions: https://www.justice.gov/atr/merger-guidelines/applying-merge...
So those businesses either know, or expect, that either:
a) these guidelines will be changed in a way that makes them hard or impossible to revert (i.e. through legislation or a Supreme Court judgement); or
b) there is little risk of a future change of administration.
Or (c) that any future administration is going to have a lot of more pressing concerns that will drown out seriously relitigating past mergers and acquisitions, and any concerns they do have will most likely be mollified with agreed remedies that sacrifice far less than the value of doing the merger.
Very few administrations do everything they theoretically could under the law and their own guidelines (even the ones that also do lots that violates both.)
Well there's also a c) - Whatever they get away with now they will have in pocket, and whatever penance they will have to do with a future administration will take years and years of legal back and forth to actually pan out, by which time it will be watered down so any fine will dwarf the profits made during this period.
Also, if they manage to reach "too big to fail" status by that point, whatever punishment will be nothing more than a slap on the wrist.
So those businesses either know, or expect, that either:
a) these guidelines will be changed in a way that makes them hard or impossible to revert (i.e. through legislation or a Supreme Court judgement); or
b) there is little risk of a future change of administration.