You forgot the big input: how many other lessors are offering properties. There isn't a dial that owners can turn to just raise rents in a 2-sided market.
You're quite right. The trouble with cartels is the incentive to cheat by offering a better price under the table is very hard to resist.
For example, auto dealers often will sign terms with the manufacturer saying they won't sell a car below a certain price. But the dealers can (and do) "cheat" on this by throwing in extended service plans, below-cost financing, a higher price on your trade-in, etc.
A common theme on HN is that people cannot negotiate prices. They can, all the time. All that's needed is to try. Even just saying "is a discount available" will often get you 10% off. (Of course, you have to be nice about it.)