Even for normal long-term rentals, some landlords own enough properties that it is more profitable to hold some of them off the market to raise prices than it is to rent them all out at lower prices. So it's also a product of the concentration of the housing rental market. I seem to recall seeing not too long ago a "market intelligence" product that basically allowed landlords to collude with each other to do this, except the collusion was through a service and algorithm that provided plausible deniability with regard to price fixing.
