Because you can use your position to promote expensive listings with more profit. A truly functional aggregator would lower prices by creating efficiencies. It's more profitable to let the market utilize price discrimination as long as the favor is to the supply side. If there is an oversupply then the aggregator would work to raise prices and would be more profitable. Right now the most profitable renter favoring model would be an aggregator that charges renters a fee. The problem is the same people don't rent frequently enough so customer acquisition costs would likely be too high.