You're assuming they were the first kind of fools rather than the second kind.

> Leaving units unsold to maintain agreed-upon price floor despite insufficient demand: collusion, market manipulation. Congratulations, you are a cartel.

Which is the thing that doesn't work when many other landlords aren't in on it, because then they get to fill their vacancies while you're stuck with the empty units.

Notice also that antitrust prohibits attempts to monopolize. Fools are quite capable of doing something which is still a violation of antitrust laws despite being ineffective and unprofitable.