In cities with zoning constraints, developers are the only natural predators of landlords. They are entirely different skillsets, building new units requires managing an extremely difficult political process for permits, raising capital, working with contractors and labor, etc. Being a landlord requires very different analysis and skills.
There may be a few landlords with the capacity to cross over to development too, but it isn't many of them.
If you have a bunch of left wing groups who are anti-development fighting to keep supply constrained even while complaining the rent is too damn high and the character of the City is changing and yadda yadda yadda, who are you[1], as a landlord, to take the wind out of their sails when you are profiting from their protests?
[1] The general “you”, not you specifically.
If landlords can make a killing by constraining supply politically, they will. So the primary cause is the incentive system and the political environment which makes it possible for them to do that.
It's all about how the reward functions are designed. So of course if you reward scarcity and profiteering that's what you get - as a secondary outcome, not a primary cause.