A monopoly is not necessary, just enough influence to affect policy. The biggest landlords are some of the most abusive and manipulative, and engage in much lobbying. Their collusion is part of why housing costs so much, and the government refusing to investigate it is due to their lobbying. Literal rentseeking lol, but yes I’m aware the word isn’t usually so straightforward.

I like this definition:

Rentseeking is an economic concept that occurs when an entity seeks to gain added wealth without any reciprocal contribution of productivity.