That’s not a cartel, that’s a free market. It’s not as if two or three landlords run the show.
Predatory pricing, also known as undercutting, is a pricing strategy in which a product or service is set at a very low price with the intention to achieve new customers (Loss leader), or driving competitors out of the market or to create barriers to entry for potential new competitors.
https://en.wikipedia.org/wiki/Predatory_pricing
I should have called it "price gouging" or perhaps "profiteering".
https://en.wikipedia.org/wiki/Price_gouging
There isn't really a great word for long-term exploitation of an easy structural advantage, though. Landlords don't have to form a cartel, since (a) everyone has to buy from them, (b) land is limited in supply and (c) the barrier to entry is high. Maybe it's a "natural cartel".
In my opinion there are two issues: #1: Land owners are not taxed for the un-earned value of their land. Making lots of money for just owning a piece of land is absurd. Normally this doesn't matter due to how large the USA is, but you can see its ugly result when land is very constricted like in major cities.
#2: Land owners are very limited by what they can build on their land by local laws and such, which prevents development of high density buildings, which would lower rents due to the increased number of units and efficiency at which these can be built. A land tax would even necessitate this sort of construction so that it would be profitable for the landlord to even keep the land.
So a cartel doesn't have to be any particular size.
It’s funny how people can observe cartels in other basic inputs into the economy, like energy, but miss them entirely for the equally basic input of land/housing.