Monopolies aren’t illegal per se in the US if they were acquired lawfully. What’s illegal is using monopoly power in a prohibited way. However, collusion (even tacit collusion) can be illegal even if there’s no monopoly by a single company, e.g., market sharing[1].

Note: Not a legal expert, this is just my understanding.

[1] https://en.m.wikipedia.org/wiki/Dividing_territories