Monopolies are not illegal, anti-competitive practices are illegal; there's a subtle difference.
You're right. Duopolies aren't great either and just because two companies are in fierce competition doesn't mean everything is good with the law. Both companies might be engaging in illegal anti-competitive behavior.
It's also the FTC's responsibility to challenge mergers that would substantially reduce competition.
No one is saying the merger is illegal, just that it probably shouldn't be allowed.
There is also a skyrocketing chance they are colluding via algorithm.
This needs to be discussed more in the age of AI. RealPage was just the eye-opener. Every industry that should be competing on prices has a cottage industry of "price recommenders" that is de-facto collusion in front of our eyes.