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.